Can't find the quotation, but early on he was very clear on Amazon having focussed on books, for what seemed like very good reasons. As I recall, the point was that there were humongous numbers of titles--far more than any physical bookstore could stock; there was a well-structured database of them--Bowker's Books In Print; shipping size and weights were manageable; and there were straightforward and fairly speedy mechanisms to get any book in print from any publisher--you or I might have trouble ordering directly from a publisher, but a modest-sized business like a bookstore or like Amazon did not.
As I recall, he said that it was much more suitable business than CDs, I think because the number of books in print was far higher than the number of CDs "in print."
He gave what SEEMED like a very convincing case for books being uniquely suited to Internet commerce. I remember being very surprised when they branched out into consumer goods.