I finally plucked up the courage to begin contacting publishers about the book. All the advice seemed to say 'Don't email publishers, they don't like it' and 'Publishers will keep you waiting weeks for a reply, so be patient.' Wrong on both counts.
I began by searching 'The Writers' and Artists' Yearbook' for suitable publishers and made a list. Then I checked out each of their web sites and eliminated a few. There were several which gave email addresses for initial contact. So, using the advice from the yearbook, I composed an enquiry letter and emailed it off with my Introduction, which contains a synopsis, and a chapter list with brief details. Most non-fiction publishers expect a proposal, and you will only write the book once they have agreed. I made it clear that the book is already written and substantially edited.
So yesterday morning I sent the emails and went out. When I came home I had a reply from one publisher asking to see the book! So today I printed it out and wrote a cover letter, and it will be posted tomorrow!
Watch this space.