To distract me from all the scary weather flying around, I thought I'd pass on a request. Scott
Erik Eric Kaufman at Acephalous (whose name I constantly misspell - sorry, Scott!) has undertaken the ambitious task of producing a list of "The Best Introduction to..." (fill in the blank with literary subject of your choice). For instance, The Best Introduction to New Historicist Literature or Literary Theory: New Historicism and Other Old-Fashioned Topics, Brook Thomas. I'm sure there are lots of things for the literati to argue about regarding the current choices, but the real point is: Scott needs medievalists! Well, okay, he also needs Britishists, and Early Modernists, and Classicists. Basically, he needs people who study the old, hard* stuff to weigh in with their choices. So head over there and help him out!
Me, I'm just a lowly historian, so I'm not in the best position to contribute. And I'm not sure that I necessarily agree with his periodization (I don't mind Anglo-Saxon, Early Medieval, or Late Medieval, but I'm not sure that the Twelfth Century Renaissance really merits its own category, and what exactly is plain old "Medieval" - if the others are modified, shouldn't this be High Medieval or something? But then, literature and history often follow different periodizations. And I'm a historian, so professionally required to get cranky about periodization).
I will say that from a historian's point of view, for a good general overview to English literature from the Norman Conquest through the fifteenth century, I really really like The Cambridge History of Medieval English Literature, edited by David Wallace - it's not literary theory, but it's a great collection of essays. For late medieval, I'm partial to Paul Strohm - probably a good place to start is with Theory and the Premodern Text. And while I haven't read it cover to cover, if I were trying to tackle high medieval English lit, I'd probably pick up Alastair Minnis's Medieval Literary Theory and Criticism c.1100--c.1375 : The Commentary Tradition. (I'd be amused to see lit folks' responses to my list!)
*That's sure to piss someone off!