This is a really interesting book. It's an amalgam of the Sherlock Holmes movie "A Study in Terror" and a modern day (1960's) story which is typical Ellery Queen: Ellery is trying to finish a book before the publisher's deadline, and life keeps interfering.
Ellery is working on his latest mystery novel and he's having writer's block. A casual friend shows up with an envelope for him. The friend had been at a party and someone had slipped this envelope onto the front seat of his car. The envelope was addressed to Ellery Queen, so the friend brought it to him. The friend doesn't have any idea who gave him the envelope.
When Ellery opens the envelope, he finds a manuscript purported to be written by Dr. John H. Watson! It is the story of Sherlock Holmes trying to catch Jack the Ripper. Ellery suddenly finds he is not much interested in the novel he's working on anymore. He compulsively reads the manuscript from beginning to end -- and from his chair he deduces not only who sent him the manuscript and why, but also that the story that Watson tells is not accurate. That is to say, it's totally accurate as to what Watson saw and heard, but not at all accurate as to the conclusions he came to. Holmes was purposely misleading him besides, so Watson thinks the killer is one person but it's really another. Ellery figures it all out and goes to tell the person who sent him the manuscript who the Ripper really was. (And by the way, if you've already seen the movie, the book switched things around a bit, including the identity of the killer.)
So it's clever; a story within a story, and it gave the guys who wrote Ellery Queen a chance try their hands at a pastiche! Holmes and Watson are pretty true to character and the only thing I don't believe is that Holmes would mislead Watson at the very end. I can see him not telling Watson much while they are chasing Jack the Ripper, but once it's all over, I don't understand why Holmes would "lie" to Watson, if only by omission. Watson was privy to everything that Holmes was...I mean, Holmes was always saying, "Anything you can tell me, you can tell Watson." So why wouldn't he let Watson know the truth? That's the only thing I found to be out of character as far as the pastiche goes.
Of course, the story is written as if Holmes and Watson were real people. At the end, Ellery is talking to a very old woman who had actually met them when she was a child. He tells her he's quite jealous that she actually met them. She shares her memories of them with Ellery, and danged if I wasn't jealous, too!