Well as of right now the Dungeon Master's Guide is sitting at number 47 on Amazon's best seller list.
Are those rankings based on the calendar day, past 24 hours, what?
Amazon does not make its algorithms public, so exactly what their rankings mean is a bit of a mystery, beyond, "we say this is doing better than that, in some sense".
Hmm...so basically it means...nothing. I mean, of course it means something but if the algorithm is based upon sales within the last 10 minutes, Joe Vanity Press could be #1 if his friends and family all buy his crappy "masterpiece" at once.
Hmm...so basically it means...nothing.