If you are writing a proposal for a non-fiction book that includes any kind of "How To" such as
How To Rope An Alligator And Other Reptiles;
How To Defang an Agent And Other Literary Reptiles;
How To Survive And Thrive As An Intern At Fineprint Lit
one of the FIRST things you need to do is research what books already exist on the subject.
Go to Amazon, or publishers of the kind of book you propose to write and make a list of titles. Figure out the ways your book is better, faster, stronger, smarter. What NEW and IMPROVED information or strategies do you offer that these other books don't.
Example: "In my proposed book How To Survive and Thrive as a FinePrint Intern, I have surveyed all surviving interns through Summer 2009 for tips and tricks of the trade. I have also added information on all the agents that have joined FinePrint in the last year. This NEW and IMPROVED information will offer more current, more usable advice than previous books on the subject, the last of which was published in 1996 and sold six gazillion copies."
Obviously not to be used verbatim, but you get the idea, right?
Include that information in your query. You don't need all the titles, but you need a paragraph that covers the information.
The reason you need this is because the FIRST thing I do when I get a non-fiction query is research the competition. Your job is to tell me why your book offers something more than what's already out there.
This does NOT apply to narrative non fiction (by authors such as John McPhee, Tracy Kidder, Melissa Fay Greene etc) or to memoir.