I have re-read Lord of the Rings 11 times in 8 years...I consider it my "cleansing" novel. If I am worn out and don't know what else I want to read - I'll plow through that one again. I get something new out of it with each re-read and grow to love it more and more each time.
I have a number of books that I have read more than once and that I will always keep around in case the mood strikes me. The beauty of my reader is that I can now have them in my pocket at all times in case I need them