The first tip is that readers expect books to be exhaustive on their subjects. That doesn’t mean they want the books to be long — it means that they expect that you will cover all the basic ground that needs to be covered to understand the subject, even if they know some of it already. This piece of advice may or may not be relevant to your subject. In the book world, people want to see you mention the stuff they already know, at least in passing (or to knock it down)– otherwise, how can it claim to be a book on the subject? It’s worth taking that point of view seriously.
This is a basic piece of advice, but it can’t be overstated when you’re trying to go from magazine-length to book-length writing: hone your outline and then cling to it as a lifeline. You can adjust it in mid-stream, but don’t try to just write your way into a better structure: think about the right structure and then write to it. Your outline will get you through those periods when you can’t possibly imagining ever finishing the damn thing — at those times, your outline will let you see it as a sequence of manageable 1,000 word sections.