Someone asked me the other day: How often do I reread books?
Let me break the answer down into fiction and nonfiction:
There are so many novels out there. Only a handful of them are so compelling that I would read them a second time. Being trained in Economics, I always think in terms of opportunity cost. Reading books that I've already read burns time that I could spend reading new ones.
I do occasionally reread a novel if a.) I read it many years ago, and b.) I want to experience it again in the context of the intervening years.
That sounds like something that an English teacher would say. (In fact, my English teacher did say this, back in 1985). So let me elaborate...
Just as you change over time, so your experience of a novel can change. As you age, you will be able to find new ranges of meaning in old books. You can therefore sometimes benefit from rereading books that you originally read ten, twenty, or thirty years ago.
Consider novels that deal with midlife issues like the loss of youth, the ennui of longtime relationships, etc. You might have trouble relating to some of that when you are seventeen or eighteen. I certainly did.
This obviously implies that rereading is most beneficial to older, more experienced readers. I don't see the benefit in rereading a book unless a significant amount of time has elapsed. Otherwise, you are kind of like those teenaged girls who watch the Twilight movies over and over again. If you are a young reader, you are best advised to read as widely as you possibly can. Save rereading for later on.
I began my voracious reading habit in high school (1982-6). Some of the books that I read as a teenager and have recently reread (as a 40-something) include:
King Rat (James Clavell)
A Separate Peace
A Farewell to Arms
The Red Badge of Courage
The Dead Zone
This Side of Paradise
The Great Gatsby
You'll notice that the above list contains a mixture of popular novels and classics. Many of the classics on the list were school assignments; and I probably read some them more hastily than I should have.
There are many practical reasons to reread nonfiction books. We all forget subject matter over time. This is why I sell or give away most of my novels, and hold on to most of the nonfiction books in my personal library.