I'd go further than @Greg suggests. Actually write a chapter of the book. The publishers I've worked with will want page count estimates. Having written a chapter will mean you can formulate your estimates well. It also shows the publisher you're committed. A few hours or evenings to build a book proposal is pretty trivial to the entire project.
Which means make sure you're doing the book for the right reasons. Few technical books pay enough to give up the day job.
I'd go further than @Greg suggests. Actually write a chapter of the book. The publishers I've worked with will want page count estimates. Having written a chapter will mean you can formulate your estimates well. It also shows the publisher you're committed. A few hours or evenings to build a book proposal is pretty trivial to the entire project.
Which means make sure you're doing the book for the right reasons. Few technical books pay enough to give up the day job.