Snorri Sturulson's prose Edda is a textbook for poets in the classic Nordic style. It is three textbooks, in fact, for there are three subjects Snorri sees it as essential for the skald to master, and devotes a section to each of them. It is unclear whether Snorri Sturulson wrote the entire text or merely compiled it, but his editorial hand definitely unites the text, and scholars are generally in agreement that the final third (on verse-forms) is his own composition.
Snorri, a Christian living in early 13th-century Iceland, introduces his book with an explanation of how the old heathen legends are fictions derived from a misunderstanding of history of the Aesir, a tribe of men who, he believes, migrated to Scandinavia after fighting in the Trojan War. These men were falsely worshiped as Gods by the people of northern Europe, and the myths invented about them are recounted in the first section of the book, the Gylfaganning. This section is presented with a frame story about one Gylfi being told all the legends by three men purporting to be the god Odin, allowing Snorri to tell the tales without endorsing them as fact.
This is followed by the Skaldsaparmal, a text on the construction of kennings, choice phrases which use legend to allude to a thing or a concept without naming it outright. Such phrases are a major tool of the poet in the tradition documented in the Edda. Afterward, the final part is the Hattatal, a discussion of verse forms current in medieval Iceland.
The latter two parts tell us how a medieval scribe conceived of the task of writing poetry, and how verse was understood by the popular poets of the age. Taken together, they provide a more thorough understanding of the poetic arts of their era than could ever be gleaned merely from analysis of extant works. The first part tells us a wealth of information on what Norse myths had survived (and in what form) even after two centuries of official Christianity. The stories show a slightly Christianized slant, with a more diabolical understanding of Loki than in the very earliest sources, a more kingly Odin who seems to have taken on certain characteristics of God the Father, and a Christlike Baldur, but over all they preserve the old legends quite faithfully, giving details which find confirmation in texts of an earlier era. More complete than prior sources, the Gylfaganning serves as one of our best texts on Norse myth, like a nine-hundred-year-old regional Bullfinch's.
The Everyman edition of Snorri's Edda, translated by Anthony Faulkes (1987), gives the text in the form of a readable and easily-accessible handbook, flowing as smoothly for a modern reader as the original must have for Icelanders of its day. The edition also adds supplemental material putting the Edda in its historical context, allowing the reader to approach it with a closer understanding of its place in Icelandic literature than the text alone provides. Because of the source's importance in the understanding of Norse myth and composition, it's an important work to have in such an accessible form, and Faulkes rose admirably to the task of providing it.