I see that others have already posted good summaries of the
process of approaching a publisher. I don't have much to add
at this level of generality, but if you have some specific
questions, I may (or may not) be able to answer them. I've
published two physics textbooks, one at the graduate level
and one at the upper-division undergraduate level. Both
were published by Addison-Wesley, though the grad-level book
has been spun off (with its friends) to a company called Perseus.
I also know the physics editor at Wiley, if you're looking
for contacts.
There are some warnings that I would give to anyone who is ready
to negotiate a contract. If you want to change any of the
standard contract language at all, it helps a lot to have
more than one publisher who is interested in your book.
For me, the main issue was the price of the book, and by
getting two publishers to bid against each other (and making
a few concessions myself), I negotiated a price that's half
what they would have otherwise charged. For other authors the
main issue may be something else, but as my editor once told
me, every author has something.
Feel free to call or write with more questions.
Dan Schroeder
Physics Department
Weber State University
dschroeder@cc.weber.edu
801-626-6048