If you don't already know the answer to which book you want to write then the question you're asking is "which book do you think will sell". The answer to that

If you don't already know the answer to which book you want to write then the question you're asking is "which book do you think will sell". The answer to that question is "who cares". No one wants to read yet another tech book written by someone who is not both qualified in and passionate about the subject.