Tiger, I think you might really enjoy Wolf Girl by Theresa Tomlinson - I read it a couple of years ago and loved it.
Another one that might appeal if you like historical crime fiction is Morality Play by Barry Unsworth - it's very unusual and very, very good.
There's also a marvellous Tom Marlowe trilogy by Chris Priestley that consists of Death and the Arrow, The White Rider and Redwulf's Curse. I adored them all when I read them. Again, they're historical crime fiction - a little Sherloch Holmes-y.
Then there's the wonderful Endymion Spring by Matthew Skelton which is a bit like a Da Vinci Code for teens.
Set in Stone by Linda Newbury is excellent, but you might want to leave that one another couple of years as it's aimed at the older end of the teen market and deals (very tactfully) with some adult themes.
The Rainbow Bridge by Aubrey Flegg is actually 2nd in a trilogy, but also works well as a stand-alone work (I've not read the other two in the series).
Orphan of the Sun by Gill Harvey is set in ancient Egypt and is very well-written, as well as being a very good story!
Hitler's Canary by Sandi Toksvig is set during WWII and is surprisingly good. I wasn't expecting such a riveting read from this comedian as I'm not a fan of hers, but she's a much better serious writer than she is a satnd-up comedienne!
The Cleopatra Curse by Katherine Roberts is an action-packed tale set in ancient Egypt - it's fast-paced and exciting from start to finish.
The Medici Seal by Theresa Breslin is another intriguing mystery.
Ithaka by Adele Gerasis the tale of The Odyssee told from the point of view of the women left behind.
The Angel Stone by Livi Michael jumps back and forward between modern-day Manchester and the same city in 1605.
Hearts of Stone by Kathleen Ernst is set during the American Civil War and focuses on a fmily torn apart by the war.
Witch Child and Sorceress by Celia Rees are both very, very good and deal with a young girl accused of witchcraft near Salem.
The Book of Dead Days by Marcus Sedgwickis another mystery and is packed full of twists and turns!
The Star of Kazan by Eva Ibbotson is gorgeous - I loved this one.
The Spur on the Plate by Maureen Rylance is a short novella set in Scotland in 1529.
I'm pretty certain you'll enjoy most, if not all, of these ones, Tiger. And in a few years time, I'd really recommend Simon Scarrow to you - his Roman Legions series is excellent.
(And although I've linked all the titles to my reviews of these books, you don't need to worry about plot spoilers - have a look and see if any of them appeal to you!)