How do I determine how many books I have in my Books library?

Enter iBooks into a Spotlight search, and find your iBooks folder from the list Finder presents (it'll be buried under your hidden Library folder), and select Get Info in Finder on that folder, and you'll get a count.

It'll likely be the following path:

~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks

FWIW, I agree. Seems like a simple request to fulfill. Why not let Apple know by providing Feedback for Books.

Apparently you have to count them yourself. The best way that I have found is to select Books>View>View As>List.

Nice tip...seems like a no-brainer for Apple to incorporate into the Books UI.