Also make sure that you check the publisher's website to see if they have guidelines. For example, McGraw-Hill Professional (their self-help section) has a very specific list of what they want to see in a query and proposal, in a specific order, and the EXACT content they expect to see. If it doesn't meet their guidelines, they won't even look at it. They'll just return it with a stock rejection and you'll never know if they might have liked it or not!
Here are the McGraw-Hill guidelines, so you can see what I mean about what they want to see. It's extensive, and if you miss one detail, they might well return the query.
Project Submission Guidelines
McGraw-Hill is committed to maintaining the highest standards of quality in all areas in which we publish. If you have an idea for a book that you would like us to consider, please simply provide the information requested below. This will assist us in evaluating your proposal. Summarize your material concisely and accurately, being as specific as possible. Remember, the quality of the proposal you submit may be our only guide to the quality of the book that you plan to write.
Rationale
Why do you feel compelled to write this book? Why will someone want to read it? Is there a particularly timely nature of the subject area? What are the specific benefits of your book? These will be key selling points, so be precise.
Subject
Describe the contents of your book in commonly understood language. Be as precise as possible, providing both a general overview and a rundown of subjects treated in detail. Indicate how in-depth your coverage will be.
Market
Who will be the audience for this title? Try to avoid falling into the "all things for all people" trap. Specify who will need to read this book, citing job titles, and identifying industries. Include information on professional associations, potential courses, and any other items that may help us reach your audience.
Competition
List other books on the same or related subjects that have been written for the same market. Include all pertinent information (author, title, publisher, date published, price, and number of pages). Then provide a sentence or two to explain how your book is different from (and of course, better than) each.
The Book
Describe your ideas about the physical book: How many pages do you estimate it being? Approximately how many illustrations will be included? Can these be black and white, or is color necessary? How long will it take you to complete the entire manuscript? Is this tied to any software release? Will the work require any add-ons such as a disk or CD-ROM?
Your Curriculum Vitae
We'd like to get to know you. Please include a recent resume, as well as a list of professional affiliations. Are you a member of any Associations related to the subject matter of the book?
Suggested Reviewers
While we may not use them, at times we find it helpful to have the names of one or two people whose expertise or reputation in your field will facilitate our evaluation process. These should not be close colleagues or friends, but peers whose opinions you would appreciate having. Please provide names, addresses, and phone numbers if possible.
Rough Outline, Book Materials
Ideally, we'd love to see a finished manuscript. Therefore, please include as much material as you have already prepared, including Table of Contents and any Chapters you may have. If pertinent, it's also a good idea to enclose some illustrations if you can.
Thank you for your time and effort in compiling this information. We hope that the preparation of this proposal has helped you think about your book, and increased your awareness of publishing requirements.
*********
So, in the case of this particular publisher, a proposal is IN PLACE of a query, but that's not always the case. Each publisher is different, and sometimes the website is the most recent information. Not always, of course -- some publishers update their website once a decade whether it needs it or not!
