E.E. Wardale 
An Introduction to Middle English [PDF ebook] 

First published in 1937, this book supplies a history of the living growth of the English language from Old English to the medieval period. It offers an in-depth study of the growth of vocabulary through literature and social interaction, bringing out the fact that it is chiefly words that foreign influence has affected — leaving sentence structure almost unaltered. Isolative and combinative changes in phonology, the accidence of nouns and plurals, pronouns and adverbs, and verbs are also examined in detail, along with a general overview of the features Middle English and a brief outline of each dialect’s most striking characteristics.