Posted By: 
Max WederLee
Prior to the internet and ebay, I had lots of difficulty finding these.  In the past years, they have become easily available and the price has dropped accordingly.  I don't have the circulation numbers here at work, but they were published weekly in the millions.  "Burt Standish" is the pseudonym for Gilbert Patten, a Maine author who was the creator of Frank Merriwell and his younger brother Dick.
Hundreds of baseball and other sports covers exist, as well as non-sports.  I believe a listing of the baseball titles are available in Andy McCue's fine baseball fiction bibliography Baseball by the Books .  Here's a link to a site which provides more details http://www-sul.stanford.edu:9001/dp/owa/pnpack.draw?pid=122
Hope this helps
Max