How to Write a Book Proposal?

You have a lot of options on how to learn this. I recommend Writers Market and their tips. Also checking the websites of the publishers that publish your genre to learn what they want in a proposal ( some do include notes on this). If you try to gear the proposal to the publisher's interests and works published it will tell the editor you have done your research and will be a team player. My genre is scholarly and professional so it may not be the same. But I do want to add that as an editor a proposal reveals a lot about your writing skills, your discipline, your organization and your talent as a writer. You not only sell your book to us you sell a working relationship.
Good Luck!
Talkwrite

Are you planning to write fiction or non-fiction? As talkwrite says, there's a big difference between writing a non-fiction "proposal" and a fiction query and synopsis. They're totally different animals. So, let's make sure we're all talking about the same thing first.

Also right here on AW- read the FAQ's threads. There is great advice from people who know.
Talkwrite

I'm not sure what kind of proposal you're interested in writing, but I've got these two books sitting on my desk with stickies sticking out of them:

• Nonfiction Book Proposals Anybody Can Write by Elizabeth Lyon

• How to Write Attention-Grabbing Query & Cover Letters by John Wood.

There's a terrific book out there that I recommend to everyone. It's by an agent named Susan Rabiner, and it's called Thinking Like Your Editor or How to Think Like Your Editor. It's geared toward scholarly writers who want to reach a broader audience, but the advice applies to everyone. One caveat: I'm not sure I'd write a book the way she says--matter of style and taste. But you can't beat her approach to writing a proposal, and I speak from experience; her advice helped me sell my second book.