The Death Collector by Justin Richards
I found this mystery a little unusual. I don't know if I would clasify it as science fiction, fantasy, or both. Throw in the fact that it takes place in 19th century London, and you have one super cross-genre book.
This is the story of a pickpocket, an industrialist millionaire, a vicar's daughter, a knight of the Brititsh Empire, and a lowly museum clerk. The story begins when a young pickpocket named Eddie picks the wrong pocket.
The pocket that Eddie picks belongs to George who works at the British Museum. Two of George's collegues have just been murdered over some journals belonging to a scientist. George has in his possession the last surviving page of the final journal.
I don't want to go into too much detail, but the book does involve mechanical men and dinosaurs. To discover what else happens to Eddie, George, and the rest, you will just have to read The Death Collector by Justin Richards.