Other considerations:
I am not going to directly contradict Jenna, but I'd like to throw in some comments based on my own experiences.
Book Completion It's true, for a nonfiction title you don't need to have the manuscript finished before you start seeking a publisher. But to expand on the above statements,
you had better make sure you can finish a whole book on the subject before you start pitching it. Chances are, you're married, you have a full-time job, you might have kids. You already have a busy life. A nonfiction book will require research: libraries, bookstores, interviews, driving across town to meet interviewees, seeking source materials, etc. Do you have time to do all this? If you submit a proposal that says you plan to have a completed manuscript in six months, then you had better be sure you can follow through. You'll have no idea what kinds of obstacles you will face between the idea stage and printing out yet another manuscript revision.
Proposals Don't get too hung up on the mechanics of a proposal. Nonfiction publishers will sometimes, more often than not, tell you exactly what they want in a proposal. Sometimes, with noses pointed high, they might refer to a proposal as a "Vita." Needless to say, it's the same thing. Follow their guidelines and write a good one. But yes, nonfiction publishers require them.
Source materials Make double sure you will have access to source materials: photographs, letters, legal documents, interviews, before you begin your project--even before you send out that proposal. If you plan to write a book about the subway in your city that was built but never used, you'd better know where to find the supporting photographs, city documents, and maybe even legal clearances to use some of the materials. Some private collectors can be surprisingly stingy when it comes to sharing rare photographs (yes, I've been there) so don't depend on just one source for information or materials (which I do not). For instance, you might say in your proposal, "The Historical Society has all the photographs and documents I need." Then after your contract is signed, you discover that either the Historical Society has just a few instead of the hundreds of images you thought they had, or they DO have the images, they just charge $100 apiece for publication rights. If you secured an advance, that money may completely disappear just for this purpose. So, plan to use multiple sources.
Briefly on Advances Not all publishers offer advances. Be forewarned that some do and some do not. An advantage to not getting an advance is that you're guaranteed a nice royalty check every six months.
Manuscript length Each publisher has different requirements on text lengths and photo layout. You may imagine your book will be a nice coffee table book with big, glossy pages. What you end up getting may be different than what you expected. Be flexible. Also, you will be required to modify your text and proposed image layout to fit the requirements of the publisher. If you've written 75,000 words and have 300 photographs, but your publisher's books only contain 128 pages and 200 photos, then be prepared to edit to their standard. Don't complain about it and just do it. Your book will be better for it. Trust me, I've been there.
On Writing Yes, you should read Stephen King's
On Writing, but that's not what I mean here. When you write your manuscript, depending on its subject matter of course, write it for the general reader. Have you ever picked up a book published by a university press written by a professor who overuses long, ponderous words in complex sentences? I've tried and I can't get through two paragraphs of it. Just a suggestion: keep it simple.
Last but not least check, double check, and triple check your information. There's nothing worse than having readers point out all the inaccuracies in your book after it's been on the shelves for a few months. (Note, this did
not happen to me in either of my first two books. However, it did happen to one of my publisher's other books that I know of.) Although you may already have planned to make use of beta readers, have an expert or two read your manuscript NOT for grammar and puctuation, but for accuracy. Give them an acknowledgement and a copy of the book for their trouble. Although you can bend the laws of physics to work in fiction, you can't do the same in a nofiction book. Your book could be used a reference book in a classroom, or will be an aid to researchers. Accurate information is a
must, and believe it or not, some authors don't get that.
There's more, but my lunch hour is up.
edited to add: To find out how I wrote my first book, please check out my
faq page.
allen