Regarding hardcover and paperback books

Think of it like movies- they go into the movie theaters first to maximize profits, then gets a boost when the dvd comes out.

It's dual purpose really. Hardcovers are much more profitable ($25 dollars vs $9.99 for paperback). It’s to a publisher’s advantage to sell as many as possible. So the hardcover is released, and everyone willing to spend that much money to read the book will do so, because it allows them to read the book sooner than everyone else.

That first group of readers then serves as marketing buzz. People review the book, they talk about it, they get a lot of people really excited about reading it. By the time the paperback comes out there is a readership waiting for it, which not only makes for good sales, but concentrates initial paperback sales into a short period of time, which maximizes the odds of getting onto bestseller lists.