Hi Miss Snark,
I have a handful of technical non-fiction credits to my name, and I'd like to move more into mainstream non-fiction, but I'm not quite sure if the querying differs. For fiction, I understand, you want your manuscript complete, edited, and ready. For non-fiction, which has considerable cost and time requirements, is the process the same? For technical writing, usually a query consists of no more than an outline and pointers to other publications.
With several topics in mind, I don't know if the best use of time is to pick one that I like, and start off on it, or begin querying for interest first.
Querying non fiction is much different. You need to consult some of the very handy books on How to Write Non Fiction Book Proposals. You can find them in your library, or the writing reference section of any bookstore. There are zillions of them. You will have to cough up a sample chapter though; it's not just an outline and a list of competing titles.