Various 
Routledge Library Editions: Chaucer [PDF ebook] 

Reissuing works originally published between 1964 and 1994, this superb set of books is an array of scholarship on one of the most important authors of the medieval period. Some of these titles are introductory books on Chaucer and his works but others are specifically focused on his humour, or the sources he drew from, or his importance to the development of English poetry, and between them they address all of his works, not only the Canterbury Tales. A good coverage of critical study in the area of medieval poetry that contains interesting fodder for any literature student or academic.