After a Chaucer seminar, I have never looked back. As an English major, I reference Chaucer so many times a day that I can't believe I went so long without really understanding his work. A must have for any serious Chaucer scholar.
Fantastic compendium of Chaucer, including Troilus & Criseyde, the Parliament of Fowles and The Canterbury Tales. Knowledge of Middle English is a must.