Hitchhiker's Guide to the Galaxy, get all of those books. They are all great and funny books. However, I never really got the last one. Maybe it was because I was reading them when I was younger.
Ender's Game, is a great book, a very dark one, but it's a very good read. You could also get its sequel, Speaker for the Dead, but its ending really did make me feel disappointed. I'm currently in the process of reading Xenocide, and then, I'll probably read Children of the Mind, just to see how it all ties together. But seeing how things are going, I'd only recommend the first two books.
The Harry Potter books, you might not like 1-2 but after 3 it all just keeps getting better.
The Adventures of Sherlock Holmes, I have the complete collection and I read through the first two novels, and they're looking very good.
As someone else has said, I also recommend Watchmen even though it's a graphic novel, it's written better than most of the books today.