[japhyr]

I would encourage you to figure out a clear plan for how you want to get this work published.

If you're hoping to go with an established technical publisher, they usually want to see a proposal rather than a finished draft. That gives them a chance to develop a shared vision, and then execute on that. It also lets you see if an established publisher agrees with you that this is a good idea for a book.

If you're planning to publish it independently, then developing a specific plan for how you'll share it would be a good thing to do now rather than later. You might look at Leanpub or Gumroad, or maybe as a technical person you're thinking of building a homegrown site; it's not unheard of.

Any of these will move you out of the unvalidated state of this work and into a state where you're forced to demonstrate to yourself and others, including your partner, that people want to see this book completed and are willing to pay for it.