The CCITT Blue Book was published in 1988 and is the most recent publication with a composite series of Recommendations on all standards aspects of ISDN.
The I 100 Series deals with the general terminology employed for describing ISDN.
The I 200 Series deals with Services.
The I 300 Series deals with Network Aspects of ISDN.
The I 400 Series deals with the User Interface.
The I 500 Series deals with Internetworking issues.
The I 600 Series deals with Maintenance issues.

The work of the CCITT has now been reorganised and is being carried out by another ITU organisation called the ITU-T. The Recommendations produced by the ITU-T are published when they are available and not, as in the case of the CCITT, as a composite set of Recommendations published on a four yearly basis.