Writer's Digest has a lot of great books. One I particularly like is called Mystery Writer's Handbook. (It may be out of print, but you can probably get a copy through ABEbooks.com)
Writer's Digest also has a whole series of terrific resources for getting information for writing mysteries. Their titles are:
Deadly Doses (guide to poisons)

Armed and Dangerous (guide to weapons)

Missing Persons (finding abducted and missing people)

Murder One (guide to homicide)

Private Eyes (guide to private investigators)

Malicious Intent (guide to how murderers, robbers, rapists, etc. think)

You can't tell I'm interested in writing a mystery myself someday, can you?
