I have found sometimes after reading a particularly good book, a heaviness washes over me and I cannot begin reading another book.  Here are a few ideas on how to cure the "book hangover":

1. Switch it up.  If you always read historical fiction, try a murder/mystery.
2. Try a beach read; something light.
3. Ask your local librarian for a recommendation.  They are always willing to help.

The most important thing is to continue reading!