What next for my Y7 avid reader?

The Greengage Summer - I've just found it in a box and still love it.

Oh, also I Capture the Castle, The Changeover, and Madeleine L’Engle’s Austin family books.

Penelope Lively?
Beverley Nichols?

Ifigoup, your list is almost exactly what I was about to suggest, apart from Antonia Forest and Ruth Rendell - it’s spooky! I would add When Marnie Was There, Playing Beattie Bow, Keeper of the Isis Light, ( sounds dodgy now but nothing to do with terrorism!), Charlotte Sometimes, Howls Moving Castle, Witch Week, Anne McCaffrey’s Pern books, all of which aI enjoyed at that age.

Agatha Christie
Isaac Asimov robots stuff
Other vintage sci-fi: Chocky, The Lotus Caves, The Tripods, Day of the Triffids, The Midwich Cuckoos
Alan Garner, e.g. The Owl Service
Susan Cooper’s The Dark is Rising series
Antonia Forest, starting with Autumn Term
I got into Ruth Rendell/Barbara Vine at that age, but perhaps a bit dark for right now

Cider With Rosie - my top suggestion if she's not read it, it's lovely
Dorothy L Sayers
The Shipping News
Miss Smillas Feeling For Snow
The Curious Incident
Lorna Doone

This thread is a treasure trove that my DD will love!
She's reading Percy Jackson at the moment. She loved HP and the unladylike murders series.
She also adores Miss Peregrine's School for Peculiar Children. There's now 5 books in the series.

A couple more to add to the list...

The 'What Katy Did' series might appeal if she's enjoyed Little Women.

Agatha Christie - DC has just finished 'Death on the Nile' and is working through some others.

I enjoyed 'Not a Penny More, Not a Penny Less' at that age.

Does she have any hobbies/interests - are there any autobiographies that she might enjoy?

From the adult to children's spectrum you've mentioned, OP, has she read Jane Eyre and Robin Steves' Unladylike Murders?

Daphne du Maurier?

Lots and lots of really good suggestions on here.

DD is in daily contact with her very supportive school librarian who has been invaluable because she has suggested books that will feed into the curriculum, expand her knowledge of key topics across subjects and has provided useful talking points for her to consider after each book. She also sends lots of ideas just for fun and pleasure but she is a brilliant, dynamic librarian who is instrumental in nurturing dd's abilities in a way I (an avid reader myself and ex academic librarian) simply couldn't do. For example, dd is reading quite a lot of philosophy and factual research alongside certain historical texts and trying to tease out quite complex ideas about why and how we read. Don't forget other forms of reading too. Poetry in myriad forms, prose, discursive essays, epistolary novels, modern forms (dd has just read a book written in the form of multimedia and electronic communication), plays and so on.

Does your dd have a similar librarian at her school?

If you have a kindle, introduce her to Project Gutenberg and let her roam freely through the out-of-copyright classics.

Second Edge Chronicles.

DS1 (August Y8) has just finished Orlando.

How about Neil Gaimans Norse mythology book? Ds really enjoyed that, also Mythos as previously mentioned.
Hitchhikers Guide to the Galaxy went down well, as have the Robin Hobbs books.
Has she read all the Ann of Green Gables books? Usually very cheap on Kindle
DS enjoyed Jasper Ffordde at 12 - depends on your DDs sense of humour

What about the Wolf Brother series, or The Edge Chronicles?
I have 4 children and all of them enjoyed those at a similar age.

My y7 DD has been into Agatha Christie recently.

https://www.amazon.co.uk/dp/1783446730/ref=cmswwrwaaapiii_NkN4EbFVC5QVP Things A Bright Girl Can Do

I haven't actually read it, but it came highly recommended here on MN. I gave it to dd when she was about 14 IIRC, and she has loved it so much ever since, that she is now using it as her English Lit A-level free choice text.

Quite random, I know, but Tim Peake's book about being an astronaut is v accessible and very interesting.

Patrick Ness, John Green, Rainbow Rowell?

Yy- my DD loved My Family And Other Animals at that age.
My children love Sherlock Holmes too, and Hitchhiker's Guide.
My youngest is a huge Pratchett fan- start with the Tiffany Aching books, the Wee Free Men is first.

The Book Thief
The Shadow of the Wind
The Lie Tree
Oranges Are Not the Only Fruit
This Thing of Darkness
Pride and Prejudice

Tarka the Otter
Born Free series by Joy Adamson
Hitchhiker’s Guide to the Galaxy
Diary of Adrian Mole
The Da Vinci Code
Agatha Raisin
Catcher in the Rye
To Kill A Mockingbird

Maybe some Charles Dickens- A Christmas Carol or Oliver Twist. Wilkie Collins - The Woman in White had me gripped at about that age! Some Sherlock Holmes is always a good choice imo. Then things like Noughts and Crosses by Malorie Blackman, Harry Potter (if she's not read already), Jacqueline Wilson, My Family and Other Animals by Gerald Durrell.