For anyone else interested, I highly recommend reading through Rob Fitzpatrick's "Write Useful Books":

https://www.amazon.com/Write-Useful-Books-recommendable-nonf...

It isn't about technical books exactly but it does speak mostly on how to write recommendable non-fiction, some of which can obviously be technical. It's got a lot of practical advice for the actual "doing" of the book, and has inspired me to at least start dipping my toes in.

For people in this thread, I strongly recommend Rob Fitzpatrick's book on how to write a book as a business, "Write Useful Books: A modern approach to designing and refining recommendable nonfiction":

https://www.amazon.com/Write-Useful-Books-recommendable-nonf...

Even though I'm not an author I found the advice very useful, it can trivially be applied to building and marketing any product.