Methinks I'll give you the same suggestions as I gave Aubi, simply because they're too good to miss.
I'll also second JD's suggestion of
The Weirdstone of Brisingamen. I read it years and years ago, and I still get it out from the library to read again. Great book.
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An absolutely cracking YA trilogy is the
Bartimaeus Trilogy by Jonathon Stroud:
- The Amulet of Samarkand
- The Golems Eye
- Ptolemy's Gate
It's always the one mentioned when people ask about good YA books, and it is definitely worth checking out.
Another, is
The Wardstone Chronicles by Joe Delany:
- Spooks Apprentice
- Spooks Curse
- Spooks Secret
The fourth book is being released this year, if it hasn't already been.
Let's see.... there's Eoin Colfer's
Artemis Fowl series, and both of Anthony Horowitz's Alex Rider, and
Power of Five series. Erm... oh yes! Garth Nix! You can't go wrong with Garth Nix - two series: one that is currently being written, called The
Keys to the Kingdom, and a trilogy that has been out for a few years, consisting of, in this order,
Sabriel,
Lirael, and
Abhorsen.
Those should keep you busy for a while.