The Prose Edda, or Snorra Edda, is a wonderful manual for Icelandic poets written at a time when first-hand knowledge of Norse mythology and poetic technique was fading. Alongside long sections on poetic diction, which are included only selectively in this translation, are the most authoritative prose versions of the great mythological stories. Without Snorra Edda, we would have little understanding of the web of story, because the poems themselves are so allusive that many of them are hard to understand without first knowing what they are about.
The Penguin Classics translation is functional rather than inspirational. Even so, its up to date language and conservative translation approach make this the most generally accessible Prose Edda translation I've come across.