amnesia of content of a book

I was traveling and expected a problem getting to sleep so I read for 3 hours. Sometime during that period, realizing I was not getting sleepy, I took an Ambien cr. Several days later I went to review the book because I couldn't remember how it ended, and I had no memory of anything I read ! I spent three hours re-reading it and it wasn't even familiar. Nothing like that has ever happened to me and it's frightening. I thought maybe I had a stroke, but now I'm thinking maybe it was the Ambien.