I listened to an audio recording of 'The Bartimaeus Trilogy' at work and thought they were wonderful. The first two books were about a young wizard who was 12 and 13 (if I remember rightly) and the third when he was 17 and working for the government. He was n't a wizard in the same way that Harry Potter was, it was more to do with chalking symbols on the ground and calling up djinnies from the past. I think the fellow who read it was part of the reason I enjoyed it so much. He was a cockney and did the different voices. Though I have to warn you it does not have a happy ending, which suprised me for a book aimed at teenagers.
i'd say the percy jackson series, it's quite similar to harry potter, except it's with demi-gods not wizards, it features the same sort of creatures (centaurs, unicorns, that sort of thing) and is based around magic and myth
The book to read is "Tom Tullet and the Saxon Ruby" This book is a truly exciting story and a good read. Tom is a typical 21st century teenager from a poor single parent family. Bored to tears by the lack of stimulus in his life, he entertains himself by committing petty and mischievous crimes. When he is finally excluded from school and has to stay with his grandmother in the tiny village of Saxonflete where opportunities for mischief are few. A ruined medieval castle looms over the village and it is while he is exploring the ruins that he is thrown back into the year 1193, where a series of dangerous and exciting adventures ghange his whole attitude to life forever