Nice one
Have a look at this thread for ideas:
If only I had more time
If it’s going to be quite an extensive book with just one or two authors, why not charge a small fee? Something like $5? Or if you really want to keep it free, have you thought about contributing to the Elixir School project? Reason being is I think writing a largish book takes quite a bit of work (and motivation!) whereas contributing smaller amounts where there are lots of contributors takes less.