Hello there,

To learn in details about Middle English Poetry I suggest you to read David Daichess' A Critical Histroy of English Literature. If you want quick information you might as well look for essays in JSTOR, which is an online library for students. Andrew Sander's book in History of English Literature will suffice too. Cambridge Guide to History of Middle English Literature has some very student-friendly essays on Piers Plowman, Sir Gawain and the Green Knight, Pearl, Purity, Paitence, etc.