Today I went to a large bookstore which I had heard has a very good selection of English language books. I was in heaven, as I am running out of books to read.
A while ago, there was an article in the local newspaper about why so few Japanese speak English, or other languages, and one of the reasons was that (almost) everything that has ever been published has been translated into Japanese, thereby negating the need to learn a language in order to read about a subject originally written in another language.
I have yet to see a Japanese book in hardback so, therefore, stores can get many more books on their shelves. Whether books are all paperback for this reason, I don't know, but the store I was in today was jam-packed with thousands and thousands of books on four floors - the 4th being the English-language books.
I've also noticed that when people read books in public - and so many read on the Metro - every book has a paper cover, another sign of one's privacy in this crowded society I would suspect. So, when asked at the checkout if I would like paper covers on my books, of course I said yes.