Dear Miss Snark,
I'm interested in your comment that "The only thing an editor really cares about is how well it sold." It makes perfect sense -- after all, publishing is a business.
I've heard that selling 7,500 copies of a non-fiction book is considered "successful," although it seems pretty low to me. My first book has sold 2,500 copies in the first six months. What is the lowest sales level that would get an editor's attention? I have no idea what "typical" sales are for a first non-fiction book or at what sales level a publishing house would consider a book profitable enough to be interested in a second book.
Ah yes, this is the question that makes us all crazy.
It's impossible to answer of course, because books vary so widely, as do publishers expectations.
The key here is "expectations". If you exceed expectations, you're going to be ok. It's like coming in second in the New Hampshire primary...it's a big "win" if you were expected to trail the field, or you came closer than anyone thought you would to the guy who came in first.
And likewise, you can sell a million books and have furrowed brows at your next editor lunch cause they thought you'd sell five million.
I've had novelists sell 16,000 copies and be disappointed, I've had novelists sell 5,000 copies and be ecstatic.
Selling 7500 copies of a non fiction title if you only expected to sell 500 is a good thing.
Frequently I'll include how many times the novel had to be reprinted if I'm pitching the next book to a bigger house: "we sold through three print runs, in excess of 7500 copies, it was hard to keep the thing in stock" kind of thing.