As a member of the Pepsi generation, I have a few suggestions from a
younger reader. I wrote a book proposal as a final exam project for a
technical writing class. It was just a letter to (any) publisher, an
introduction and one chapter, the subject being saltwater aquaria
highlighting the "For Dummies" aspect. The prof gave me an A (sound
of my tendons creaking as I slap myself on the back).
To echo JOAT's comments, a conversational tone, rather than
instructional makes for a much easier and faster read. It was easy
for me to get caught up in a preachy milieu, and in keeping with the
intent of the project, my editor (the prof) called me out on it a
couple of times.
A la Gandalf, at least design, if not create line drawings, rather
than photos wherever possible to cut production costs. This can also
help illustrate particular aspects of, say, joinery than photos
I never shopped the proposal to a publisher (I was way too concerned
with drinking beer and having fun at the time), but my prof thought it
had real promise.
Good luck, and keep us updated.
This advice is worth what you paid for it :)