I started Ex Libris yesterday, a collection of essays. It turned out I'd read the first essay before. I thought it must have been in another book, or online somewhere. Then I'd read the second and I started to get worried. The next one or two were familiar, but I couldn't be sure I'd read them, but by the time I was halfway through, I knew I'd read the book before. Unfortunately, I didn't keep track of my reading before I started this blog, so I don't know when I read it, although it vaguely seems like it was around the time I read 84, Charing Cross Road.
Anyway, Ex Libris is an excellent book and highly recommended. Even though I didn't remember reading the book, I remembered and still think of several of the essays.