io9.com lists exactly what the title above says, Six Astounding Young Adult Novels of the Pre-Potter Era.
"The success of Harry Potter has established that the young adult market in fiction can be insanely lucrative, as have other successful scifi series like Philip Pullman's His Dark Materials trilogy and Scott Westerfield's Uglies series. Now traditionally adult scifi authors like Cory Doctorow and John Scalzi have released YA novels, and publishers promise more smart YA fiction is on the way. But this is hardly the first time that YA fiction in the scifi genre has flowered: in the 1950s, Scribner's did an entire Juveniles series, with over a dozen novels devoted to teen space adventure (including some of Lester Del Rey and Robert Heinlein's most beloved books). But these weren't the only cool kid scifi books of the pre-Potter era. We've got six more great, old-school YA books for you to rediscover or read for the first time."