Delete hidden books from iTunes
SELF ANSWER:

On Windows, books should have been available from the iTunes application but, for whatever reason, they weren't for me. So, I ended up moving my iTunes library back to a Mac running OS X 10.11.

On OS X, it appears that all books functionality has been moved from iTunes to the iBooks application. I opened iBooks and it immediately offered to move books from iTunes. I then deleted the books from iBooks. After closing both iTunes and iBooks, an inspection of iTunes Music Library.xml shows that all the books "tracks" have been removed from my iTunes library.