You only read an average of 4 books a month?!?!?! I can never understand how it takes some people a whole week to read just a single book.
Yep, a humongous number of people here (like me) can easily read 20-30 books a month. Those books do need to come from somewhere.
Considering ereaders are sold at cost,
not below, ebook sales are money. So really, a lot of people each buying a few books is just as profitable for Amazon as fewer people buying more books. But either way, there are definitely people who buy more books.