Sci-fi? Oh boy, here we go. I've read so much of that, I can't remember all the good ones.
Hitchhiker's Guide to the Galaxy is a classic, of course.
Kenneth Oppel's Airborn/Matt Cruse series is more steampunk, but the later parts - particularly Starclimber - are definitely sci-fi.
Pearl North's Libyrinth was fun, especially since the setting involves a giant library (hence the title). It's more about a love of knowledge (and books) than anything, wrapped up in an interesting future world away from Earth.
Enchantress from the Stars, by Sylvia Engdahl, doesn't seem to be very well known, but it's definitely worth a read. The protagonist is from an interstellar, hyper-advanced society, and has to help solve a problem on a world technologically still in the medieval era, with tech so advanced it can be passed off as magic. Arthur C. Clarke's third law hard at work.