Marion Zimmer Bradley was a science fiction fan before she became a famous author, so this early work unsurprisingly feels a lot like some of the early guides to SF books that were published by fans in the early days of the genre. But here we are looking at a very different kind of fantasy!
Worth browsing through if you are at all interested in the subject, but hopelessly outdated if you're looking for a guidebook.
This collection does not contain thirteen short stories and none of them feature John Thorndyke.
Half the book is taken up with the title story, a lively romp in which the hero nearly loses his job at the National Gallery in London after a valuable painting is stolen by ingenious thieves. When the painting is returned, the authorities are willing to forget the whole matter -- but he wonders why anybody should go to such trouble to steal the painting and then return it.
The resulting plot wil have you engrossed.