The number of exclusive titles in the Kindle library has grown to 180,000 from 130,000 in April, Amazon said.

The significance of the new Amazon data is that nearly all these books are available for owners who also have a Prime membership.

The Prime membership program -- $79 a year after a free, one month trial -- allows borrowing for free, up to one title a month.

Analysts say Amazon's line of e-readers and the $199 Kindle Fire tablet are likely profitable partly because of sales of online products. The Prime offering not only allows borrowing of books, but also instant streaming of movies and TV shows. Members get free shipping of thousands of products on sale.

Kindle-exclusive titles include "War Brides' by Helen Bryan, which has been purchased or borrowed 270,000 times, and three titles by Karen McQuestion that have been purchased more than 500,000 times, Amazon said.