They are the basic sources of our knowledge about Viking mythology, and there are two of them, readily available in English translations.
The Poetic (Elder) Edda is a collection of Old Norse poems, mythological, apocalyptic and heroic. It's the older of the two, and the poets are anonymous and unknown. Read it closely, and find the names of Tolkien's dwarves!
The Prose (Younger) Edda is the work of the medieval Icelandic poet (and historian and politician) Snorri Sturluson. It is a how-to manual for skalds and includes in one of its sections, the "Tricking of Gylfi", our best prose source for Norse myth.
Stop reading bad summaries written for kids -- go to the source today!