Baugh, Albert C. and Thomas Cable. A History of the English Language. 4th ed. London 1993. (traditional standard history of the English language, very readable)
Blake, Norman (Ed.). The Cambridge History of the English Language. Vol.II: 1066-1476. Cambridge 1992. (very detailed information by specialists)
Burrow, J. A. and Thorlac Turville-Petre. A Book of Middle English. 2nd ed., Oxford 1996. (good and short survey of the language on the first seventy pages)
Crystal, David. The Cambridge Encyclopedia of the English Language. Cambridge 1987:30-55. (excellent overview, richly illustrated)
Lexikon des Mittelalters. München/Zürich 1980ff. [Einträge s.v. 'Englische Sprache' und 'Mittelenglische Sprache'].
Markus, Manfred. Mittelenglisches Studienbuch. Tbingen 1990. (typically German production)
McCrum, Robert et al. The Story of English. Lonsdon 1986. (book publication to the BBC series with the same title)
Mossé, Fernand. A Handbook of Middle English, transl. by James A. Walker. Baltimore 1966. (standard introduction to Middle English in the 1950s and 60s; grammar and anthology)