A book proposal is a business plan for your project. And you are not just the author, you are the star of the show. Your name will be on the book, you are its spokesmodel.
Parts of a typical book proposal:
• Short description of the book
• Your bio and credentials to write this book; other books and published articles by you, include any reviews of your work
• Book outline (chapter listing with brief descriptions)
• Sample chapter
• Who's your audience? (Be specific, do some research)
• Who's your competition? (List similar books and talk about what makes yours different/better)
• Marketing plan (your ideas for how to market, who to market to, what you will do to support the book once it's published)
• Bullet-pointed list of all the main points of your proposal (a cheatsheet or summary of the proposal)
• Optional: cover ideas, layout ideas. You can include comps if they are professional looking.
Make sure you proofread everything several times. Typos, grammatical errors, and awkward phrasing will be noticed.