Sony says their eBook store has more than 1.2 million titles but if we discount Google Books, the actual store size is around 60,000 titles.
Barnes & Noble’s eBook store too claims to have more than a million books for the Nook but the number of titles available in the store is around 26,000 – the rest are public domain (out of copyright) works that you may download through Google Books.
Apple’s promotional material says that “tens of thousands” of book titles are available on their iBookstore but the exact numbers are unknown. However, a simple Google query reveals that iBookstore is the smallest of them all with a collection of around 22,400 titles.
On the plus side, iBooks does support the ePub format and therefore you may download any of the public domain books from Google Books and read them on your iPhone or iPad.
Comparing the Size of Online eBook Stores
Related: Buy Books Online for Less