Hm. There are books I reread often, like several of LM Bujolds books, CJ Cherryh, Lee&Miller, Modesitt, because I find new angles and depths and connections in them every time. Then there's those I start to read but realize I've read them before and they're not good enough to reread. Then of course there are the books I never finish, can't reread those!
Finally there's the smallest group of all, those that are "read-once, rememeber-forever", Tigana by Guy Gavriel Kay for instance. They made such an impression the first time through that I only need see the cover to remember the plot and the pain and the despair and the tragedy and the beauty, so I never need reread those, it'd be too exhausting emotionally.