I've heard that books have their heyday and then they stop selling, or sales slow down. Others have said that you can earn an income from a book for years. I don't see why a good book can't sell for a long time. But I can imagine sales slowing down, for one reason or another.
Let's say you've got a book out there and sales slow down a bit. Is it ok to refurbish it? i.e. restructure the content, add or take away some bits, rephrase certain parts, giving it a new title and cover, and so on. Basically reinventing it.
Is that ok? Do authors do this?