Or, what goes into a typical query package: three chapters and an outline of the novel in question. Plus the cover letter, of course.
It shouldn’t really be necessary to say that when we’re talking about “three chapters” what we mean is “the first three consecutive chapters” and not some random collection of chapter highlights . . . but the conversations I’ve had with slushpile readers have convinced me that yes, it is necessary. (No, not for you, of course . . . but there’s always somebody who doesn’t yet know the customs of the community. And we were all of us clueless once.)
Likewise, by “outline” we don’t mean the I-II-III/A-B-C/1-2-3/a-b-c format that our high school teachers sweated so hard to insert into our resistant brains. What “outline” means, in this context, is a five to ten page synopsis of the novel in question, usually single-spaced, giving the main arc of the plot, the important characters, and something about the setting and general milieu of the story. If there are important plot twists and revelations, mention them here; your potential agent or editor is not worried about spoilers. Customarily, in an outline, the plot is narrated in the present tense — rather as though you were telling a good (and non-spoilerphobic) friend the story of this really nifty movie you saw last night.
Writing an outline is not fun, at least not for most writers. The best way to get through it, I find, is to grit your teeth, tell yourself “It’s not an art form, it’s a sales tool,” and push on through.
As for cover letters — briefer is better, generally. Include the title and word count and a short description of your book (“a cozy mystery featuring a retired card sharp”), relevant publications if you have them (“three short stories in Ellery Queen’s Mystery Magazine“), and relevant personal information (“I made my living for twenty years as a Mississippi riverboat gambler.”) But the single most important thing you can put into your cover letter is your return address and telephone/email contact info. There’s nobody quite as sad as an editor who has found a good manuscript . . . and has just discovered that the title page with the author’s address on it has gone missing.
Don’t make an editor cry. Include a cover letter with your full contact information, even if all that the letter itself says is the prose equivalent of Roses are red/Violets are blue/This is a book/That I’m sending to you.