This one, first published in 1931, is the sixth in the series (although I have seen references to it being #7), and Lord Peter has been firmly established by this point. Here, while on a fishing holiday in Scotland, Wimsey takes part in the investigation of the murder of an artist. As the title suggests, there are six solid suspects, only one of which is the actual murderer, leaving five red herrings. By far, most of the book is devoted to Wimsey, and several members of the local constabulary questioning the suspects and constructing possible scenarios for how the murder was accomplished.
Interestingly, the end of the book details a number of the official inspectors and police personnel offering very plausible scenarios for whodunnit but, of course, Lord Peter Wimsey points out the flaws in their logic and offers the correct solution.
I found the novel to be a little tedious in places; just so many detailed clues and potentially misleading information offered by the suspects that I lost track of it all early on. Had I been a proper literary sleuth myself, I would have prepared a complex spreadsheet to record all the clues offered, who was where when, and who saw them there, and who was providing alibies for whom. Everything right down to the timetables of the trains which would prove valuable evidence indeed. But I am too lazy to do that, of course, so I let Wimsey handle that for me.
But the conclusion was most satisfactory. I will not let another ten years pass before picking up my next Lord Peter novel.