Hi, toasty!
Yes, the guidelines will give you your answer, but only if you understand the lingo. I think that's where you're confused, so let's go over that a bit:
1. Types of Letters:
Query Letter: Also called a "letter of introduction" as you indicated, a query letter is sent to introduce both you and your book. A query letter is sent IN PLACE of a synopsis, asking the agent/editor for permission to send a synopsis, partial or full manuscript. This letter should be no longer than a single page, with letterhead at the top or bottom that states your name, your mailing address, city/state/zip (or city/province/postal code), your e-mail address and your phone number. It should include a greeting that states the person you're trying to reach (such as, "Dear Mr. Smith), a paragraph stating what you are asking for from the agent/editor (such as, "I would like you to represent my book/publish my book.") Another paragraph or two should then "introduce" your book. There are any number of ways to do this, but the primary elements you want to include are the length, the genre you believe the book fits into, and a BRIEF description of what the book is about (remember it's only one page.) The letter should close with your request for them to respond, that you're including a SASE and your signature.
Cover letter: A cover letter has all of the elements above, EXCEPT a description of the book. A cover letter has your contact information and encloses a synopsis, a partial manuscript or a full manuscript. The only purpose is for the agent/editor to later contact you.
2. A Synopsis: This is a 1-5 page detailed description of your book. Some people single space and some people double space it, but it should not be more than five pages single spaced UNLESS the guidelines tell you differently. You may, however, make it shorter if you're able to get in all the information in a shorter page count. Again, you want to state the size of the manuscript and add your name (or your pen name) in case the synopsis gets accidentally separated from your cover letter. Concentrate on the main plotline and name the PRIMARY characters. It also helps if you provide a brief description of the characters such as age, hair/eye color, race/species, etc. I personally like to add the setting, so the agent/editor knows whether it's set in the present, past or on another planet.
3. A Partial: When a guidelines page or a letter from an agent/editor requests a "partial" manuscript, they're normally looking for the first three chapters of the book, or around 50 pages. Most editors consider a Prologue to be a "chapter" for purposes of a partial.
4. A Full Manuscript: Just what it sounds like. They want to see the whole book. If it's not done, tell the agent/editor that so they can decide whether they want to look at the 80% you have done, or if they would prefer to wait until it's complete. It's a case by case thing.
Hope that helps!
