40% of All Titles Were Mysteries

David Bordwell has an interesting post, I Love a Mystery: Extra-credit reading, that has the following:

Contrary to what historians imply, the puzzle novel with a brilliant sleuth was far from defunct. Christie’s Poirot and Sayers’ Wimsey retained their fame into the 1940s, significantly outselling Hammett and Chandler. Ellery Queen’s novels are not read much today, so it’s hard to imagine a time when over a million copies of them were in print. More generally, the public’s appetite for mystery novels and radio plays was intense. In 1940, 40 % of all titles published were mysteries, and in 1945, an average four radio shows devoted to mystery were broadcast every day, each drawing about ten million listeners.

I find that figure stunning.

Though he lists sources at the end, Bordwell doesn't indicate which of them contained that figure. I'd be curious to know how it was arrived at. It would be remarkable if it were 40% of all fiction titles, but 40% of all titles without qualification? I don't know what to make of that. How many titles were published, and who read them? What was the ecology of entertainment like at the time?