The CEFR publication in English

Council of Europe. Common European Framework of Reference for Languages: Learning, Teaching, Assessment. Cambridge University Press, Cambridge, 2001.

available for download at:  http://www.coe.int/t/dg4/linguistic/source/framework_en.pdf

The original study 

North, Brian J. ‘Developing Descriptor Scales of Language Proficiency for the CEF Common Reference Levels’. In Common European Framework of Reference for Languages: Learning, Teaching, Assessment: Case Studies, edited by J. Charles Alderson, 87–105. Strasbourg: Council of Europe, 2002.

North, Brian J. The Development of a Common Framework Scale of Language Proficiency. Peter Lang, New York, 2000.

North, Brian, and Günther Schneider. ‘Scaling Descriptors for Language Proficiency Scales’. Language Testing 15, no. 2 1998 

Schneider, Günther, and Brian North. Fremdsprachen Können – Was Heisst Das? Zürich: Rüegger, 2000. (contains an extensive chapter in English describing the calibration process)