I've read "Three Men in a Boat" by Jerome K. Jerome for the first time when I was about ten years old. In the past 20 years, I've read it at least 20-25 times more, and each time I find myself rolling with laughter, as if it were the first time.
"The Hobbit" is also a magical book, for me. I've read a dozen of times, and the magic never goes away.
As an adult, I find that I don't re-read many books. I feel that there are so many things to read, and so little time to read them, that I can't afford the luxury of going back to books. However, I did read "The Good Soldier ?vejk" 3-4 times in the past years, and I will definitely go back to "Love in the Time of Cholera", which I've read once, a few times in the future.
It's hard for me to say what makes me return to a book and mark it as some "cultural cornerstone" of my personality (as I, without a doubt, regard the four books' I've mentioned). I think that the common factor to all these books is a certain naivety and a fundamental belief in humanity and in the goodness that lies in the core of man-kind (even in "?vejk", that does look into the ugliness of what the wars of men are).