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What you Need to know before you hunt monsters
By an Amazon.com customer
Encyclopedia of the Undead: A Field Guide to Creatures That Cannot Rest in PeaceEncyclopedia of the Undead: A Field Guide to Creatures That Cannot Rest in Peace by Bob Curran
Buy new: $12.91 / Used from: $11.95
For the non-vampire/werewolf, this is the single best book I've come across, which is not to say that it doesn't also do a good job with those two monsters, as well.
The Vampire Book: The Encyclopedia of the UndeadThe Vampire Book: The Encyclopedia of the Undead by J. Gordon Melton
Buy new: $16.47 / Used from: $2.32
One of the most extensive books on vampire lore.
The Werewolf Book: The Encyclopedia of Shape-Shifting BeingsThe Werewolf Book: The Encyclopedia of Shape-Shifting Beings by Brad Steiger
Buy new: $14.93 / Used from: $7.98
Counterpart to the Vampire Book, almost as extensive (though there's been less written about vampires in general.
The Vampire in Lore and Legend (Dover Books on Anthropology and Folklore)The Vampire in Lore and Legend (Dover Books on Anthropology and Folklore) by Montague Summers
Buy new: $10.36 / Used from: $7.61
An older source, but with a lot of good information on the history of Vampires
Monsters: An Investigator's Guide to Magical BeingsMonsters: An Investigator's Guide to Magical Beings by John Michael Greer
Buy new: $13.57 / Used from: $1.59
From the perspective of the believer, rather than the skeptic. Even includes tips on how to track and rid oneself of the monsters.
Cryptozoology A To Z: The Encyclopedia of Loch Monsters, Sasquatch, Chupacabras, and Other Authentic Mysteries of NatureCryptozoology A To Z: The Encyclopedia of Loch Monsters, Sasquatch, Chupacabras, and Other Authentic Mysteries of Nature by Loren Coleman
Buy new: $10.20 / Used from: $7.30
A somewhat scientific perspective, dealing with non-supernatural beasties.