The books I've come back most often to are:
Belle du Seigneur by Albert Cohen (sometimes translated to English with the same title or with the title
Her Lover, apparently), a very rich book that, among other things, asks the question of why you fall in love with somebody. Each time I read it (I've read it three or four times since I was in high school and I'm now in my thirties) I find something different in it.
The princes of Amber series, by Roger Zelazny (first five books mainly), a really gripping and deeply moving series.
I had never heard of
A Night in the Lonesome October, it's now very high on my wishlist, thanks for the suggestion!
Some of Jean Giraudoux's plays, mainly
Electre and
Ondine (
Ondine was apparently translated to English with the same title, I don't know about
Electre). These plays are pure moments of deep happiness. They're funny and thought-provoking at the same time, with an irreverant touch that make some of the dialogues pure jewels.
Electre is actually one of the only books that I had to read for class that were good enough for me to enjoy, so prejudiced am I against books read for school.
The Idiot, by Dostoievski.
The books by Boris Vian are also books that really moved me and made a lasting impression, but for some reason I never really read them again. Maybe I should...
For me, one of the reasons I read again the books I loved is that I am utterly unable to remember the story once I closed the book (though I clearly remember the impression it made on me). So reading them again is a way to discover them again, pure bliss!