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Both little black books, rolodexes and classic detective movies have now been replaced by electronic databases and crappy hollywood movies.