This is a standard tool, primarily used by jazz musician, that provide a common source of music notation for performance. It is essentially a collection of lead sheets that provide the essential elements of standard songs so any combination of musicians can perform any song contained in the collection as a band. Typically, each musician will have a copy of several common fake books that will allow them to perform with a combo (slang for combination or a small band that has no specific instrumentation). The term fake books comes from the use of the book by jazz musicians where they will perform the basic melody (or head) of the song, followed by one or more of the musicians in the band perform an improvised solo using the written chord progression. The band, in effect, "fakes" an arrangement by making spontaneous musical decisions.
One critical aspect of the fake book is that it comes in several versions for different instruments. A version in "C" for piano, guitar, and other non-transposing instruments, a version in B-flat for instruments such as trumpet, tenor saxophone, clarinet, a version in E-flat for instruments such as alto saxophone, baritone saxophone, and and a version in bass clef for instruments such as double bass and trombone. These different transpositions provide the ability to create any number of instrument combinations.
These books are also used by musicians to practice and learn standard songs in standard keys. The jazz musician will also use the fake books to practice improvisation on the melodies and chord progressions of the standard songs.
The original fake books of the 1940s and 1950s have been unavailable for many years. These books were created by jazz musicians and were not sold in stores, only distributed by other musicians. The books didn't have copyright permission for the songs and typically had many errors with the chord progressions. The fake books that are available today contain legal copies of the songs and have much more accurate chord progressions. One of the most well-known fake books is "The Real Book" with several hundred lead sheets with primarily standard jazz songs. Fake books are now available with other genres of music including, Broadway, Dixieland, Latin, Rock, and Country Western, to name a few.
See also lead sheet.