Believe it or not, most Kindle books are lendable.

I have a database of Kindle books that I built last year, but I know that now there are over a million titles. So, year old data:

   > db.books.find({'lendable': true}).count();
   543519
   > db.books.count();
   846456

It would be interesting to compare that to the sales list. See if popular books are lendable.

Self-published books are lendable by default, and are subject to a lower royalty rate if lending is disabled. Most major publishing houses do not enable lending b/c they have negotiated separate contracts with Amazon that do not require it.