It seems like the people who are making money at this are the ones that have a following and sell PDFs of the books + addons (video training, a product) on their own.

Manning and Leanpub are built around being able to ship chapters as you write them - I think the feedback while working would be a huge help for motivation.

If you were choosing a publisher, I'd consider what the association with the publishers says about you (e.g. I've always been most impressed by the O'Reilly books)