August's recommendation: The Prose Edda, by Snorri Sturluson.
In April I introduced you to the Poetic Edda, one of the
great works of Norse literature. Now we move on to its cousin the Prose Edda.
You may have heard this one referred to as the "Younger Edda." It's a misnomer; this work was probably written earlier than the "Elder" Poetic Edda. But the material in the latter work is mostly older than the time of its writing, so relative dating of the two is a mess. Just skip those terms: call them Poetic and Prose.
Unlike the Poetic Edda, the Prose Edda has an author, a thirteenth-century Icelandic historian and poet named Snorri Sturluson. It also has a more specific and localized date of composition, somewhere in the vicinity of 1220. On the whole, it's a more organized work -- which is not meant as a criticism of the Poetic Edda. They're both interesting in their own ways.
Mind you, I say that on the understanding that I have never read the entirey of the
Prose Edda, have no intention of ever doing so, and would recommend you not try. Why?
Because two of its three sections are a manual for thirteenth-century Icelandic poets.
The "Skáldskaparmál" is fifty thousand words of discussion between Ægir,
god of the sea, and Bragi, god of poetry, on the nature of poetry itself. There's a
lot of mythological information tucked away in here, so it's worth looking through, but
you run the risk of drowning in eight million kennings. You know, those phrases like
"the swan of the sweat of the thorn of wounds," which is a really complicated poetic
way of referring to a crow? These two gods list off a million of those, with
explanation of where they came from. It's not something you really want to sit down
and read through, and frankly, it's debatable how many of those kennings were ever
actually used, and whether the mythology behind them was ever a matter of real belief
(as opposed to invented poetic tradition). But on the other hand, if you need a lesson
in how kennings work, Sturluson is your man.
Then there's the "Háttatal," which is a primer in how to compose skaldic poetry. Again, not so much for the modern reader. But to continue a comment I made when discussing the Poetic Edda, the meter most commonly used in skaldic poetry is dróttkvætt -- "court meter" or "lordly meter" -- which tends to be way more intricate and excessively clever than eddic poetry. Not for the faint of heart to imitate, especially since I'm not sure it would work very well in English.
But the first part of the Prose Edda is very much worth reading. That's the "Gylfaginning," the "Tricking of Gylfi," which makes up maybe a quarter of the total text. This part is straight-up mythology, and has a lot of the most famous stories you'll encounter in other books. Some of it overlaps with the Poetic Edda, giving slightly different versions of the same tales, but some of it is all-new. To a limited extent it's processed through a Christian framework -- Iceland had been Christianized since well before Snorri was born -- but on the whole the stories seem to be mostly intact.

If you go looking into the Prose Edda, the "Gylfaginning" is the part you should
read. This stuff is ripe for use: as short stories, as novels, as inspiration for an
invented Norse-like culture. You could mine an entire career out of it. Like I've
said before, Norse material has a charming bluntness to it, a forthright bloody-minded
sense of humour, and we could use more fantasy that channels it properly, instead of in
a watered-down form that loses half the flavor.
Alas, I am not familiar with many translations; in fact, I only know one. My copy is the Everyman's Library edition, translated by Anthony Faulkes. There may be others as good or better, but that one's at least decent.