Over at her blog, Beth was talking about ordering a book that she read as a kid and wondering if it would still be any good. I think it was Neil Gaiman who noted that returning to a book from childhood can be a journey fraught with peril. Sometimes they are just as wonderful as you remember them and other times they just suck.
I find that most of the books that I read as a child and that I can still re-read as an adult are, not surprisingly, books that were not written strictly for children. Much of my early reading consisted of reprints from the pulps of the 1930s, though at the time I didn't know that. That list would include Doc Savage, Tarzan, Conan, Fafhrd and the Grey Mouser, John Carter of Mars, etc. All of these are characters I still like to revisit, though I have to be in just the right mood to read Doc Savage these days.
I started reading adult level books when I was nine or so, and don't really recall too many of the young adult books I read. The one series that I did try to revisit not too long ago was Alfred Hitchcock's Three Investigators. This was a series that I absolutely loved. Brought a couple at a flea market and found them utterly unreadable.
Other series that I recall recall fondly are the Encyclopedia Brown stories and a group of science based books about a character called Danny Dunn. I remember one in particular, which involved Danny and his friends being accidentally miniaturized. Years later, I wondered if the writers of the movie Honey I Shrunk the Kids had also read Danny Dunn and the Smallifying Machine. I haven't attempted to revisit Danny, though I've seen the books from time to time on Ebay. I suspect he would fall into the same category as the Three Investigators.