The Poetic Edda and the Prose Edda of Snorri Sturluson are the most important sources of Norse mythology.
The Eddic poems were minstrel poems, passing orally from singer to singer for centuries.
None of the poems of the older Poetic Edda are attributed to a particular author though many of them show strong individual characteristics and are likely to have been the work of individual poets. Certain events in the stories date the stories as no earlier than 850 and many suggest they were composed in Norway as opposed to Iceland.
The Prose Edda was however the work of Snorri Sturluson who was an Icelandic historian, poet and politician who was twice lawspeaker at the Icelandic parliament, the Althing. He was also the author of the Heimskringla, a history of the Norwegian kings that begins with legendary material in Ynglinga saga and moves through to early medieval Scandinavian history. For stylistic and methodological reasons, Snorri is often taken to be the author of Egils Saga.

The Prose Edda

A manuscript of the Poetic Edda was written in the 13th century but remained unknown until 1643 when it came into the possession of Bishop of Skálholt. Versions of Snorri's Edda were well known in Iceland at the time but there was speculation about an older Edda, as Snorri quotes and “Elder Edda” in his sage.
The Bishop sent the manuscript to the King of Denmark as a present and the manuscript became known as the Codex Regius.
The Poetic Edda contains 29 long poems, of which 11 deal with the Germanic deities, the rest with legendary heroes like Sigurd the Volsung (the Siegfried of the German version Nibelungenlied) and details many of the stories of the Norse Gods and the heroes of Norse mythology.
The Prose Edda is a textbook for poets and discusses Norse mythology and makes reference to the Poetic Edda in order to instruct on the subtleties of poetry. Many of the familiar Norse Gods appear in the stories, gods such as Odin, Thor and Loki.
The Prose Edda consists of Gylfaginning ("the fooling of Gylfi"), which deals with the creation and destruction of the world of the Norse gods. The Gylfaginning deals with king Gylfi's encounters with the Æsir, and his disguised journey as Gangleri to Asgard. There Gylfi is ostensibly exposed to the glories of Asgard and its inhabitants.
The second part of the Edda, the Skáldskaparmál or "language of poetry" is a dialogue between the Norse god of the sea, Ægir and Bragi, the god of poetry, in which both Norse mythology and discourse on the nature of poetry are intertwined. Bragi discusses poetic language in some detail including the concept of poetical language.
Finally, the Edda ends with the The Háttatal which describes the types of verse forms used in Old Norse poetry.