Bookhounds of London is a campaign frame, by Kenneth Hite, for Trail of Cthulhu.

Your characters aren't stalwart G-men or tweedy scholars this time around, serving their country or sealing off forbidden frontiers. They're working the main chance, and selling maps (and maybe guidebooks) to those forbidden frontiers. They are book-hounds, looking for profit in mouldy vellum and leather bindings, balancing their own books by finding first editions for Satanists and would-be sorcerers. They may not quite know what they traffic in, or they may know rather better than their clientele. Peddlers of blasphemy and madness aren't nice people, and the only consolation is that their customers are worse yet.

In a Book-Hounds of London campaign, the Investigators do not investigate horror and strangeness professionally. Rather, they investigate books about horror and strangeness and become, seemingly inevitably, drawn into the horror themselves. If they could just sell a pristine copy of the 1845 Bridewell edition of Nameless Cults, pocket their 40% (or 400%) and move on, they would. But it's never that simple. Not for them. Not for book-hounds. Not in London. Not now.