Everyone then spends a satisfying number of pages chasing around after the mysterious murderer. The book's cover shows Martian tripods devastating their way along, so you may presume that aliens are involved. It's good old-fashioned pulp of all flavors -- assassins, assignations, monsters, sewer chases, airships -- circling (as all spy-story pulp should) around the mirror-hall of shadows which is loyalty among spies. The book's epigraph, of course, is from Kipling's Kim.
I won't even try to list the literary references which I noted in passing (because I only noted them in my head) (because writing them down would have eaten a lot of paper, and would not have been the point anyhow).
Do we get a grand explanation of how an alternate history came to be full of Holmeses, Foggs, Jeckylls, Westenras, Frankensteins, and so on? (Not to mention the Stokers, Houdinis, Babbages, and Mrs Beetons flipped into international spies and politicians.) Well -- no. I still think the author has something specific in mind, given the centrality of the Bookman and its copying ways. But it's not a big-reveal book; nor is it a one-grand-scheme sort of trilogy. It's a book (and trilogy) which winds its schtick to the highest pitch and then lets it sail off into the Paris skyline. As it were.