As an early and rather authoritative example of Sherlockian pastiche - the collaborators being the son and the authorised biographer of Holmes's creator - there is much to interest collectors.
The collaboration was not smooth, as Douglas G.Greene relates in John Dickson Carr: The Man Who Explained Miracles. There is some doubt about who wrote what - though at times Carr's highly recognisable style breaks through the convention of pastiching the original Conan Doyle stories.
Parallels to 'canonical' stories are uncomfortably close sometimes. The stated intention of expanding the tantalising references Doyle made to unwritten cases did not work out, and the new stories often have to abridge those references, or quote them selectively, or explain them away.