Can't see books on e 250; can see them on PC

I have an e 250. It used to work fine to copy books (.wma) to the sansa. Suddenly, the most recently transferred book in hidden on the sansa. however, when i plug it into my PC, i can see and play the files that are on the sansa. How do i get to see them on the sansa?

Why not post this question in the e200 thread?