The Blue Book is a compact disc standard developed in 1995 by Philips and Sony. It defines the Enhanced Music CD format (E-CD, also known as CD-Extra, CD-Plus and CD+), which combines audio tracks and data tracks on the same disc. The format was created as a way to solve the problem of mixed mode CDs, which were not properly supported by many CD players. The standard is not freely available and must be licensed from Philips.
E-CDs are created through the "stamped multisession" technology, which creates two sessions on a disc. The first session of an E-CD contains audio tracks according to the Red Book. As a consequence, existing compact disc players can play back this first session as a CD Audio disc. The second session contains CD-ROM data files with content often related to the audio tracks in the first session. The second session will only be used by computer systems equipped with a CD-ROM drive, or by special “Enhanced CD players”.
The second session of a E-CD contains one track in CD-ROM XA Mode 2, Form 1 format. It must contain certain specific files inside an ISO 9660 file system, though an HFS file system may also be included for compatibility with Mac OS computers. The mandatory files and directories include an autorun.inf file compatible with the Windows 95 AutoRun feature; a CDPLUS and a PICTURES directories; and an optional DATA directory.