The Duties of Servants by Jan Barnes
Borrowed this from a writer-buddy (thanks David). It's a reprint of a Victorian book describing who does what in a large household. While the footmen are out with the carriage, who should answer the door to callers? Whose job is it to set the breakfast-table in the housekeeper's room? A great resource for all writers of historical fiction.
The Crime Writer's Guide to Police Practice and Procedure by Michael O'Byrne
Another one borrowed from David. The author is a retired senior police officer. Even if you're not writing crime, this is a fascinating guide and well worth a read.