Blue Book is the name given to the Enhanced Music CD or CD-Plus (a dual session yellow/red books) format. Its name comes from the series of books, also called the 'Rainbow Books' that define all compact disc formats. The first session on the disc is a red book session and can be played on any CD player. The second session is a yellowbook session. This can be any data readable by a computer. There is almost no limit to what you can add to you audio CD release.