Kelly:
Analyzing a book's market potentila isn't that hard to do. You can start with informal research such as asking your librarian and favorite book seller what's selling in your genre. Amazon.com is another good place to find out what's out there. Search engines such as Google.com will also give you a lot of information.
The point of knowing about similar books is so that you can tell a potential publisher and/or agent what they are and how they're selling (this helps them determine the size of the potential market for yours.)the most important thing you need to know and share with a potential agent or publisher is WHAT'S UNIQUE ABOUT YOUR BOOK. Compare it to the competition. Sum up by saying something like, "While all current books in this category cover _____, mine is the only one that takes the next step and includes ______."

Hope that helps, Kelly.If you need more, see my online class, "Stalking the Markets" at www.acappela.com