''Triple Quest'' by E. R. Punshon (d. 1956), public domain. The story kicks off when private detective Marmaduke Groan approaches Commander Bobby Owen with a perplexing case.
Groan is looking for his missing client, influential art critic Alfred Atts, who vanished just before a highly anticipated Royal Arts lecture where he had promised to reveal explosive secrets. To complicate matters, Atts had expressed fears that his wife was attempting to poison him. Thrust into the complex world of art fraud and murder, Bobby Owen must untangle a web of secrets leading to a culprit whose identity remains deeply hidden until the very end.