I'm a writer, working on a non-fiction book proposal. Finding public, free sales numbers for comparable titles is nearly impossible. BookScan is a subscriber service run by Nielsen. "Above the Treeline" is a subscriber service that collects sales data from some independent book sellers. But I'm looking for reliable services that I could tap into. I saw the piece on Stack Exchange in today's NYTimes. I thought I'd pose the question. Any suggestions?
Ingram is the largest book distributor in the business. You can call their automated stock check number at (800) 937-0995. Enter a book's ISBN, and you'll get back its current sales data.
You can also get Amazon's historical data by signing up for an account at TitleZ and selecting the books you want to track.
Both of the above are free services.
In my experience, you're not likely to find much of anything useful without paying. The Association of American Publishers has some statistics for free (2009 Preliminary Estimates), but you'll have to buy the report to get anything meaningful. Is it worth $1250? Probably not in your case.
Sales figures for the publishing industry just aren't readily available. Slushpile.net ran an interesting post in 2007 on the matter, "More on Book Sales".
I can't provide a reason for the lack of availability.
You might try developing a list of similar titles and then seeing if the publishers of those titles would share at least print-run statistics.