A key element of a non-fiction book proposal is Comp Titles. These are books that demonstrate there's a market for your book and it's a market with enough buyers to make a book profitable.
You can easily find books but how the HELL do you measure the number of buyers? Unless you have access to Bookscan you're pretty much in the dark.
There is however a tiny candle in the darkness so you only have to curse half as much. There's a way to get a sense of the market (not the actual number) and it's from Amazon rankings.
I suggested this to an author in one recent Chum Bucket and she replied
What Amazon rank would be considered "good" in an agent's mind? I know that's an idiotic question, but I really have no idea. Would a book need to be top 100, top 1,000, top 100,000 to be considered a decent seller?
Well, it's NOT an idiotic question for starters. An idiotic question is "are you excepting queries?" but that's a topic for another day.
And let's refine the question to not what is considered good, but rather what the numbers can do for you.
1. The broader the category, the lower the ranking number you want if you're looking for books that sold well.
I recently needed a comp title that sent me to kids picture books. Here's the rundown for GOOD NIGHT IPAD by Ann Droyd. (you find this in PRODUCT DETAILS on the Amazon page for the book in question)
Amazon Best Sellers Rank: #1,894 in Books
#7 in Books > Children's Books > Social Situations > Sleep
#22 in Books > Professional & Technical > Engineering
#61 in Books > Humor & Entertainment > Humor
The book was published in October of 2011 and it's still pretty high up in the overall Amazon rankings (#1,894 in Books)
Amazon rankings measure how well the book is selling compared to other books sold on Amazon, not actual numbers, so you're getting an sense of velocity, not actual speed and distance.
This feels high and fast for a year old flight. That's good.
It's also doing well in some good subcategories but you want to watch out for those. Those categories can be so narrowly defined there are only ten books total in the category.
2. If the books you're using as comp titles are OLD and not selling well, use this to highlight the point that a new book in this category is desperately needed and then use the old slow comp titles to show there is still demand even for old books.
3. If you can't find any comp titles at all, you've got a problem.
A. You're delusional because you think there aren't any books like yours.
B. You're writing a book for which there appears to be no market
C. You're describing your book so narrowly you don't think anything fits.
None of these are good things.
Finding the right comp titles can take a while. Don't just pick the first three books you find. Know what you're trying to say with these comps: my proposed book fills a niche that exists; provides new insight or new information; demonstrates there's a market for this topic.