There are a lot of possibilities:
Popularity can be split by gender, which could be interesting.
You could also work on popularity by address to make a geographical analysis, but if this is a local library you probably won't get much out of that.
Another stat that could be worthwile is the average length of borrow.
But the more interesting reports could be those that combine popularity, popularity by gender, average length of borrow with the catalog attributes... so you could have a view by author, by publisher or by publication year.
That is with the current structure. If you were to add genre or media type (book, magazine, CD, DVD) attributes, you would open up a whole lot of new dimensions.