In my opinion, Roger Zelazny's best book. I don't expect that it will ever as popular as the Amber series, but it is certainly more fun than some of his more famous books.
It's a story of a special Halloween night. A few times every century, Halloween falls on the night of a full moon. On these nights magical forces can open a gateway into a dark world full of demons, ruled over by the old gods. The story covers the month leading up to this special night, as magic users from all over come together to try to influence the gateway -- some are trying to open it, others are trying to close it.
This particular Halloween is occurring in an unspecified year of the Victorian era, and the gateway is being opened (or not, as the case may be) not far outside of London. Many of the characters will be familiar to you, like the The Count (Dracula) and The Great Detective (Sherlock Holmes). A select crowd of witches and druids and werewolves are here to take part. The stakes are high, and treachery abounds.
The entire story is told from the point of view of Snuff, the pet dog and familiar of Jack. He and the other familiars help their masters by calculating the location of the doorway, performing spells, spying on the others, and trying to find out who's an 'opener' and who's a 'closer'. The familiars carefully barter information and try to increase their masters' lead over the others -- usually in a friendly manner. Never have a gaggle of talking animals been so serious, deadly, or entertaining.
The story is comparatively lighthearted, by Zelazny's standards, but not to the extent of being silly -- and of course, it's spooky at the same time. Even so, this is definitely a fantasy story, not horror. It might even pass for a children's book in today's market, although only just barely (the vivisectionists may be too much). It also has the advantage of being profusely illustrated by Gahan Wilson, whose sense of the macabre fits perfectly with the story.
The title comes from Edgar Allan Poe's poem, Ulalume. The book is full of references, both to other works (Dracula, Frankenstein, etc.), and punnish connections (the witches cat is named Graymalk, and the rat, Bubo).
A Night In The Lonesome October, Roger Zelazny, Avon books, 1993. ISBN 0-380-77141-1