Bookhounds of London expands upon a campaign frame originally presented in the Trail of Cthulhu rulebook, providing in-depth information, inspiration, and details to run a campaign centered around unscrupulous book collectors. As the name suggests it's set in London during the 1930s. Overall, this is my favorite of the campaign arcs because it's both unusual and morally-grey which makes for much more interesting story development. The PDF is very nice although this is a book that I think paying either for a hard cover copy or at least having the book printed through RPGNow's P.O.D. service is well worth the extra cost.
[5 of 5 Stars!]