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. Ideas to get you thinking are: a PDF of liner notes, band posters, a multimedia page that links to live web data (for example where your next gig is or video of the last show), games, programs, etc. . .
I wish more people knew about this possibility. There are a lot of really cool things I can think to put on a CD (and you can probably think os even cooler things) that would give fans extra inventive to invest in your art in this digital download world.