My #1 top tip for authors who are approaching agents and publishers is:
Present yourself as a professional author.
That means a one-page query letter (sometimes 1.5 pages but that’s unusual).
And, for nonfiction, a book proposal with sample chapters.
For fiction, authors should create a two-page (single-spaced) synopsis and a half-page or 1 page biography at a minimum. Also consider generating a two-page (single-spaced) marketing opportunities sheet. This should provide ideas that you have for marketing the book yourself, all things that can be done while the publisher markets to bookstores.