The "little black book" is a term for an address book, and in the simplest manner, it can merely be a small book in which a person keeps important contact information on friends, family, and business acquaintances. The term more often refers to address books that can reveal information about people’s past boyfriends or girlfriends, as well as potential dates for the future. It’s linked to romance, dating, or sexual escapades. Further, occasionally a "black book" contains the names and numbers of prominent people who have engaged in certain illegal activities, like patronizing prostitutes or escort services. Some court cases, like that of the Hollywood Madam, Heidi Fleiss, deeply concerned people who had celebrity status and were listed in her little black book.
Essentially, the book doesn’t need to be little or black — it's just a list of names and information. The more recent personal digital assistants are probably most likely to be the little black book in the modern dating/romance sense. Additionally, they’re simply practical, easy places to store contact information. The era of actually keeping a physical address book where entries are written by hand is quickly fading, as storing information becomes the work of small or larger computers. Even a cell phone with multiple capacities can be the modern black book, whatever its intent.
With smartphones, users don’t merely have a repository for addresses, names, and numbers — they also have the capacity to email or call anyone on the list directly, simply by selecting his or her name. This certainly speeds up the process of contacting people. Further, people can use some handheld devices to access not only addresses of people but also maps to their homes.
Still, some people keep a handwritten address book if they merely prefer the old manner of keeping addresses. Often, the book is alphabetized, so the user writes addresses in each section. Some of the older style books have removable sheets of paper, resembling a small binder. Users can buy more paper should they fill up a whole alphabet section and need more room to add additional addresses.
Others keep a handwritten address book of the old style because they prefer to keep information private, which is not such an easy thing to do if someone hacks into a computer. Unlike a computer of any size, a physical little black book could be burned should the owner wish to get rid of incriminating evidence. On the other hand, when the book has been a record of romantic conquests, the owner might have no need for it again if he or she is in a committed relationship.