A Night in the Lonesome October, by Roger Zelazny.
Someone once bet Zelazny he couldn't make a reader support and root for Jack the Ripper. This book was the result.
This is a story about how "a number of the proper people are attracted to the proper place in the proper year on a night in the lonesome October when the moon shines full on Halloween and the way may be opened for the return of the Elder Gods to Earth, and of how some of these people would assist in the opening of the way for them while others would strive to keep the way closed. For ages, the closers have won, often just barely... And the people would acquire certain tools and other objects of power, meet together at the designated spot and attempt to work their wills".
The Great and the Good (well... the well known, at least) of horror are called to attendance. The Good Doctor and his Experiment Man, the Count, Jack with his knife, the Mad Russian Monk, an American called Larry Talbot... Drawn by the weirdness and disruption are the Great Detective and his Companion.
Each Player in the Game has an animal familiar. This is, primarily, their story: the narrator is Snuff, the canine familiar to Jack. Through the thirty-one days, we follow as the Game progresses, and the Players seek to determine each other's allegiance: an Opener, seeking to summon the Elder Gods, or a Closer, seeking to maintain the current order.