I've thought about doing a book before. But what it came down to, was that for fiction, no publisher considered the fiction genre to be worth its weight, so it would take remarkable convincing for them to publish a fiction book. Or the writer would have to know the editor, or at least have a good literary agent.
As for non-fiction, it's not that hard to get accepted in theory, but the publisher expects you to have remarkable and amazing credentials quite often, to the point where if your credentials were that amazing, you wouldn't spend time writing them a book for a four-figure advance.