In my experience the advice about the dreaded "comp titles" you need for your query letter usually comes with a number of "must does" including to only pick books that sold well. I've never heard any useful follow on how to do that.
One thing you can do for free, if you are prepared to open a free account with an Amazon subsidiary, is look at GoodReads. Your mileage will obviously vary, but it seems probable that number of ratings (and reviews) of books is reasonably well correlated with sales activity (fun to look at series, which always seem to follow the conventional wisdom that the first book gets rated the most)
Your mileage will vary, but anything less than a thousand ratings is probably signaling not a success, and ideally you would be in the 10K++ range.
Equally 500K and up is probably veering into the cliche / best seller territory which is also good to avoid if you can.
One thing you can do for free, if you are prepared to open a free account with an Amazon subsidiary, is look at GoodReads. Your mileage will obviously vary, but it seems probable that number of ratings (and reviews) of books is reasonably well correlated with sales activity (fun to look at series, which always seem to follow the conventional wisdom that the first book gets rated the most)
Your mileage will vary, but anything less than a thousand ratings is probably signaling not a success, and ideally you would be in the 10K++ range.
Equally 500K and up is probably veering into the cliche / best seller territory which is also good to avoid if you can.
