A Guide to the Good Life: The Ancient Art of Stoic Joy by William B. Irvine
I reread Lord of the Rings, Chronicles of Amber, and Dune, at least once a year. I guess that I've read each of them at least 20 times each.
The only other book that I've read more than ten times has to be Stranger in a Strange Land.
"Bomber" by Len Deighton.
Time Enough for Love, Heinlein.
A Star Called Henry by Roddy Doyle
I've read and re-read Daily Rituals, which reveals the bizarre working habits of famous inventors like Tesla and Frederik Chopin. How I Write: The Secret Lives of Authors, is another one I've revisited numerous time.