Ah, Sophie's World is a great choice, although I found it a bit of a tough read myself. My own favorites tend to have a strong element of humor in them, though they're not necessarily light and fluffy. In no particular order:

1. The Order of Odd-Fish by James Kennedy, a ridiculously funny and clever young adult novel. Nothing has so consistently delighted me for so long.

2. The Neverending Story by Michael Ende. This is generally known for the movie adaptation, but the book is so much bigger and more thoughtful. Despite being a young adult novel, its themes are rich and complex -- I've needed to re-read it several times.

3. A Confederacy of Dunces by John Kennedy Toole, the tale of a bloated, supercilious juggernaut of a man who steamrollers his way through New Orleans disdaining everything he sees. This is such a funny novel it has reduced me to tears in public.

4. The Cyberiad by Stanisław Lem, a collection of robot fairy tales with a deep, philosophical undercurrent. This one is for the cerebral funny bone.

5. This is a bit of a wildcard slot, so here goes: Tiny Tim by Harry Stein, a rare, intimate portrait of a very troubled man. Tiny Tim is known for his whimsical ukulele covers, but this biography reveals a wildly irresponsible man-child with alien ideas about sex. Easily the best work of biography I've ever read, though the book itself can be hard to track down.