There's a tonne of stuff which isn't necessarily so easy to get hold of any more, but I have very fond memories of reading the 'Space' Anthologies, particularly Space 6 which included the classic story "Bobo's Star". I used to love Nicholas Fisk - most memorably "A Rag, A Bone and a Hank of Hair" and "Trillions"; Monica Hughes, who wrote lots of stuff including "Crisis on Conshelf 10" and "Devil on my Back". The Tripods is nowhere near the best of John Christopher's work. (He died earlier this year by the way) - but you should check out "The Lotus Caves" and the Sword of the Spirits Trilogy - which is pretty brutal stuff and targeted at adolescents. Someone has already mentioned William Sleator who died last year and who wrote the brilliant "Boy who Reversed Himself" and 'Singularity". For pure, thrilling 8-year old sci-fi action, you can't beat Douglas Hill's Last Legionary series of novels.