@afewbugs Ursula LeGuin's Earthsea and Gifts series.
Diane Duane's Wizard series, Susan Cooper's Dark is Rising series for a start. The Sabriel series by Garth Nix.

@afewbugs This rec is not YA but I see I won't be the only one. And I was reading Anne Rice vampire books at age 13 so I know it really depends on the reader!

Seanan McGuire's October Daye series ticks all the boxes: long series, strong female lead, set in the real world with fantasy elements (urban fantasy), gritty/dark with a good heart and good values buried under it all.

The MC's love interest does start out like it's going to be trope-y but it gets much better! A good dash of queer representation (though not the M.C.) Themes of found family, resisting hierarchical power structures, good stuff. There's like 20 of them and they're still going.

@afewbugs Some classics are worth revisiting like Anne McCaffrey’s dragon books. The Harper Hall starting with Dragonsong is a great entry point

@afewbugs Nettle & Bone is a great standalone book, but also fantasy.

@afewbugs Gideon the Ninth maybe? Although that is straight up fantasy

@afewbugs I’ve really enjoyed the Nevermoor (Morrison Crow) series, although maybe the protagonist is younger than you specified. Very independent and mature though.

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My 10 year old is into the "wings of fire" sequence (she is dragon obsessed). might be worth a look

@afewbugs how about the Rivers of London series by Ben Aaronovitch? They play in the now, although the oldest is 15 years old. The main character is a man, but the show is stolen by the women (and foxes in later books)
https://en.wikipedia.org/wiki/Rivers_of_London_(book_series)

McKinley’s _Sunshine_ has vampires and magic but on a very realistic world. The YA protagonist needs to keep her job, for instance. Also, excellent.

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@afewbugs my daughter and her friends like 'wood walkers', though I don't know if that will be too fantasy for your niece.

@afewbugs there's an Italian series of novels called Berlin, by Fabio Geda and Marco Magnone, my son likes it a lot, but I don't know if it's available in other languages.

@afewbugs i have a soft spot for Nausciaä the film and enjoyed 7 volume manga (which extends the story far beyond the film). I have the big 2-book hardback edition, but i think 7 separate managa books would have been better.

@afewbugs Has she read the Ruby in the Smoke series by Philip Pullman? Not modern though, late 19th or early 20th century, but a very modern feeling 17 year old female lead character. Lots of drama and excitement. The first three books are linked, the fourth picks up a minor character from the first book.

@afewbugs The Book of Lies sprang to mind, though it's years since I read it. I still remember the lively, dry-humoured teenage girl protagonist, witty footnotes & strong, hard-hitting story.

We Are All Completely Beside Ourselves. A surprising book that's hard to describe without spoilers.

'Young adult' or too adult? I'm a bad judge as it's so individual.

(If fantasy interests her some day: Terry Pratchett's Tiffany Aching + Discworld Witches books are hilarious, empowering & empathetic.)

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Perhaps the Ruby Red Trilogy?
https://en.wikipedia.org/wiki/Ruby_Red_Trilogy

Btw this thread is a gem (!), I’m bookmarking it for myself 📚

@afewbugs Laini Taylor - Daughter of Smoke and Bone,
Kristen Ciccarelli - The last Namsara
Amie Kaufman/Jay Kristoff - Illuminae
Susan Ee - Angelfall
V.E. Schwab - this savage song
Leigh Bardugo - Six of Crows

All badass female leads

@afewbugs if she'd be willing to read something older, most of the series by Tamora Pierce are excellent. My favorites were the Circle of Magic quartets (young adults come into magic, save the country, take on students of their own, and then come back together while figuring out if their friendship can survive adult traumas or if it was only a thing of childhood) and the Trickster duology (teenage daughter of a spymaster gets captured and enslaved and serves as spymaster for the slave rebellion)

@afewbugs absolutely tangentially- how about the Unbeatable Squirrelgirl?
College age in the comics, younger in the books
The Lumber Janes & Nemona were enjoyed by mine (graphic novels)

@afewbugs
Although its speculative fiction sci fi, I really like Malka Older's Mossa and Pleiti series, two strong female lead characters, mystery, romance, and academic politics... good stuff. the author is on fedi too, though I won't link her so as not to spam her.

@afewbugs Luminosity by Alicorn? I describe it as "Twilight, but Bella has more than one braincell at page 1"

@afewbugs She's probably too young for the #RiversOfLondo series but it ticks many of the boxes, if magical realism is acceptable as opposed to fantasy. It's extremely inclusive and has many young women in the series including a teenager, a newly qualified PC and a few goddesses. There are loads of books in the series and it's set in modern day London. There is some language and sex references. It includes lots of references to architecture (seriously!) and has a little dog weaving through it.

@afewbugs my 12 year old (just turned 14) loved all the Philippa Gregory Tudor books - each with a female lead. Think Catherine of Aragorn is now one of her personal heroes.
I would guess there's probably a content warning required if gifted - if you've not read any, they can be a bit racey...

@afewbugs perhaps Neil Stephenson’s Seveneves? It isn’t a series but the book is so long it’s broken up into discrete parts that would suit a book series lover’s pace. Strong female characters. Pretty dystopian since it’s about the moon shattering and destroying most of humanity. But pretty female-forward on the protagonists. I enjoyed it so much I’ve read it at least twice. https://en.wikipedia.org/wiki/Seveneves

@afewbugs I might recommend Paksenarrion series by Elizabeth Moon? Follows a shepherd's daughter (Paksenarrion) who runs away from home to join a merc company to avoid being married off.

It has a lot of good friendships in there (though also a lot of tragic happenings, including torture of the main character in the 3rd book), but no romantic relationships at all (Paksenarrion is confirmed as ace by Elizabeth Moon ... because I asked her on twitter and she said yes)

@afewbugs

Ursula Vernon/T Kingfisher has some great fantasy novels with comedy and healthy romances.

Any religious references are only to fantasy religions that don't exist in our universe eg The God Of The White Rat, which is an order that tries to solve problems of social injustice/poverty.

Swordheart;

Daggerbound;

Paladin's Grace;

Paladin's Strength;

Paladin's Hope;

Paladin's Faith.

@afewbugs my immediate suggestions are already made (Percy Jackson, Wayward Children, Discord (maybe try a non-Tiffany Aching one?)) and Earthsea probably would also be too fantasy...

Andrew Joseph White is good even if I am surprised his books are (except one) filed under YA - Spirit Bares It's Teeth is probably the least body-horror-y (see Content Warnings at https://andrewjosephwhite.com/content-warnings )

Sun bearer Duology by Adrian Thomas may also not fit https://www.aiden-thomas.com/the-sunbearer-trials

@afewbugs maybe Leigh Bardugo , the shadow and bone series?

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Going along with others here regarding the fantasy genres... It's never too early to introduce a youngling to Terry Pratchett and the Discworld series. They not just fantasy books, but very funny... and also an important place to learn about the world around you. I started reading them in my late teens and early 20's in the 90's and his books cover everything, racism, equality, trans... if there's an ism in the world... chances are terry covered it and taught people how to just be better human beings. There's about 40 books in the series too.

It's always my first suggestion.

After that, the Jim Butcher, The Dresden Files... A series of fantasy books aimed mainly at young adults and teens about a magical detective investigating crimes. Part fantasy, part detective noir.... I think there are about 15 books in the series.

Moving to more adult and into the semi sci-fi genres. The Expanse series of books... 9 main volumes and a bunch of short novellas. Set about 300yrs into our future where humans have started to colonise the solar system. Starts out part sci-fi part detective noir in book one... delves more into the sci-fi side as the series progresses... But the stories are based around real science that exists to day, along with our understanding of the universe, whilst taking logical well thought out and reasonable steps to explain advancements in tech and medicines, as well as the side effects of living in low gravity and how new cultures can begin. Might be a bit advanced for her though.

All in all... across those 3 series... must be over 60 books.

I could be way of base for suitable books for younger girls... I am after all a 50yr old man and so lack any real knowledge of what's out there for that audience these days. Terry Pratchetts daughter Rhianna also writes and has a book called Tiffany Aching's Guide to being a witch.

@afewbugs just gonna have to sweep through here as the replies roll in looking for recs for my kid. (11, and more on the fantasy end of things; I've totally lost track of what he's read already, which is a great problem to have)

@afewbugs 12 may be a year or three young, for this recommendation, but you know your reader. Naomi Novik's "Scholomance" series. https://en.wikipedia.org/wiki/The_Scholomance_Trilogy 3 books, ccollege age "kids" some sex, some supernatural monster violence. Kids die. Basically, let's do the "kids go to wizard school" trope but without the TERF-problems of author. It's dark, but really good. Strong female protagonist, with a prophecy over her head to destroy the world, that she rejects out of teen stubbornness.

@afewbugs This is a tough one and I am going to have to think about it: my usual recommendations of either Xiran Jay Zhao’s Iron Widow or Naomi Novik’s Scholomance series are maybe a little too far into speculative fiction for your niece. I’ve heard good things about Scott Westerfeld’s Uglies series but I haven’t read it myself so, pinch of salt.

@afewbugs

Rainbow Rowell's fangirl is fun, and I personally really love the spinoff series of carry on (three books) about Simon and Baz, some teenaged boy gay wizards. You might want to read some plot synopses first though!

I'm going to think more and come back later (not a kid and I don't have kids but I read a lot of YA because I'm a kid at heart)

Update: my brother has informed me that she has already read and enjoyed Noughts and Crosses, so "a bit bleak" is apparently not a concern

@afewbugs also Fly by night by Frances Hardinge (also a bit fantasy so might be out of remit)
There is a homicidal goose
The protagonist is 12 but on the run after burning her uncles barn down.
I’d recommend it to anyone actually

@afewbugs I liked the Princess Academy series by Shannon Hale
A bit fantasy/made up world (feels like a fairy tale world) but strong female lead & introduces unionising to get better conditions.
Also the Goose Girl by same author- the lead has a magical gift but there’s strong (young) woman against the world vibe (a horse does die incase that’s a dealbreaker)

@afewbugs Might be a bit old for her, but my 15 year-old daughter has been reading Sophie McKenzie novels for the past couple of years: https://www.bookseriesinorder.com/sophie-mckenzie/

(she was also into the Hunger Games)