A book of
contact details (names, address,
telephone numbers etc). Usually a small, black,
leather covered book carried by a
gentleman. Gentlemen with very large
social circles may also maintain a
rolodex of contacts with whom they correspond with less often.
In classic detective movies the detective goes through a murder victims little black book to determine their associates.
Both little black books, rolodexes and classic detective movies have now been replaced by electronic databases and crappy hollywood movies.