Edition: Penguin, 1958
Translation: J.F. Goodridge
Although I was able to read Chaucer in the original Middle English with only the help of a (fairly comprehensive) glossary, I'm glad I got hold of Piers Plowman in modern English. Judging by the excerpts given in this book, it is considerably more difficult to read, mainly because it is written in a Midlands dialect which didn't provide the basis for later literary English as Chaucer's language did.
The text of Piers Plowman is considerably more complicated than that of, say, the Canterbury Tales; there are three major manuscripts, known as A, B, and C. This translation is based on the B-text, though appendices give some parts of the C-text (which contains more information interpreted as autobiographical than the other manuscripts).
Piers Plowman is the story of a series of dreams, told in the first person by William (Langland). These dreams show in allegorical form what is wrong with the society he sees around him, and by contrast the perfect society which is to come under the rule of Piers Plowman, who stands for Jesus Christ.
One very sophisticated aspect of the allegory is that the dreams are spaced throughout the life of the narrator, and the nature and meaning of the visions changes as his spiritual understanding matures. Other than that, the book is also notable for the strong criticism of the abuses of the church current in the later middle ages. You need some understanding of medieval theology to get the most from the book, but anyone interested in the medieval world-view should find it fascinating.