One of the steps to getting your book published is to write and send query letters to try to get a literary agent or an editor at a publishing house interested enough in your book to sign you as a client, and, hopefully, offer you a nice advance — a sum of money you can use while you actually write the book.
An effective query letter has the following characteristics:
Runs one page — never longer
Contains no errors — typing, spelling, punctuation, and grammar are all perfect
Gets immediately to the point
Appears on good quality paper
Is upbeat in tone and style