Burt M. Leavenworth: Non-Procedural Programming. 362-386

Clemens Hackl (Ed.): Programming Methodology, 4th (1975) Clemens Hackl (Ed.): Programming Methodology, 4th Informatik Symposium, IBM Germany, Wildbad, September 25-27, 1974. Lecture Notes in Computer Science 23 Springer 1975  

Languages mentioned BDL {1974} High-level IBM database language for describing business applications