* word count
* audience (who will buy your book)
* book benefits (for non-fiction)
* special features
* models for your book (genre and comparable/competing books)
* a marketing position (what sets your book apart as unique compared to others like your book)
* other books you plan to write
* your platform and promotion plan (extremely important these days!)
So this all seems easy enough, right? The difficult part is making sure it doesn't all come off as boring. We want agents to read this stuff and get excited, right? This is the whole point of following our agent's advice...give them exactly what they want, so they will be excited about our books. What they don't want is boring.
WARNING! (What not to do.)
According to our other mentor M, many writers--in order to combat the fear of being boring--decorate their letters or proposals with color and Fancy Fonts and unsubstantiated (this-book-is-so-much-better-than-The-Lord-of-the-Rings!) hype. Mike Nappa wisely says, "You know what makes me want to represent a book? The book. Not your cover letter. Not your exaggerated promises. Not your American-Idol-wannabe pseudo confidence. Not your faked attempts at professional connection."
That sounds harsh, doesn't it? But it's the truth. Do you, as a writer, have something important to say? Do you have a marvelous writing voice? Then show that off. Agents like words. Give them words.
LET YOUR WRITING DO THE TALKING.