I read this when it was published, then again a few years ago. It's not my favourite Gardner (that came earlier) but it's not the worst (that came later). Flicka I liked, and Our Man was a shade less cardboard here than in other continuation novels.
I wouldn't mind giving it another read then commenting here, but that would be after I finish The Hobbit... again. (Can't escape Tolkien here, SM!)
Yes I've read all the Flemings and the continuation authors books.
So had read NSFlowers when it came out in paperback, and recently bought the new cover Gardner paperbacks.
So I just finished reading it (2nd time in 20 years) and it's a good way of absorbing the story again.
I also liked Flicka von Grusse who reminds me of the actress Elke Sommer (mit Sherman accent ! )
Because it involves a serial killer, it inevitably gets classed as a detective story.
There are some interesting characters amongst the law officers, apart from the deceptive villain David Dragonpol.
While it mentions a Princess and her sons as targets, they don't spell out Lady Diana-but it's heavily implied.
When they're moving around the castle, there are good descriptions of Dragonpol's construction of a History of Theatre with automatons in famous scenes. You can picture this in your mind.
I think the end fight stopping Dragonpol was a bit "short and sweet", over before it started!
I enjoyed it and would give it 7 out of 10.
but I'm moving on to Dr. No book now, and Quarrel's word pronounciation! hex trod nary Cap'n !