April's recommendation: the Poetic Edda.
The first year of these "primary source" recommendations focused on landmark influences in English letters; in the second year, I turned my eye to the great epics of classical literature. Now we shall head north, to the lands of the Vikings, and to the Norse poems and sagas.

We'll start with the Poetic Edda, sometimes also called the Elder Edda. Unlike the
classical recommendations of last year, it isn't a single, epic poem, but rather a collection
of poems, many (but not all) taken from a medieval manuscript called the Codex Regius. There
is no "author" of the Poetic Edda; the Codex Regius, and the other sources of these poems, are
transcriptions of oral texts that probably existed in many variant forms. And not every book
published under the title of Poetic or Elder Edda will have the same poems in it; different
editors draw the boundaries for inclusion in different places.
Because of this, it's hard to say exactly when the poems date back to. Some of them are newer, some of them are older, and of course the time when they were written down is not the same as the time when they were composed. They're old, certainly: from the medieval period, but possibly spanning centuries. What unifies them all is their style. "Eddic poetry" is fairly distinct from "skaldic poetry," and the three meters seen in this edda are, in order of decreasing frequency, fornyrðislag, málaháttr, and ljóðaháttr. (I offer this mostly for the sake of trivia, and for contrast when I talk about the Prose Edda later this year; I don't expect those terms to mean much to anybody. Short form is, eddic poetry is simpler and more straightforward than skaldic poetry, which is often an exercise in cleverness.) The language in question is Old Norse, of which modern Icelandic is the most closely related tongue.
I could spend paragraphs discussing variant editions, different poems, where the poems come from, who includes which poems . . . but it's far simpler to point you at the Wikipedia entry on the subject, which gives a nice, clear listing with both the Old Norse titles and the various ways they get translated, plus links to articles on the poems themselves.
With that technical material out of the way, what is the Poetic Edda?

It's mythology and legend, mostly. The "Völuspá," which opens the text and which
I mostly know as "The Prophecy of the Seeress," is an account of the world's creation and a
prediction of Ragnarók. Some of the poems are more narrative; others, like the
"Hávamál" or "Sayings of the High One," are more an expression of medieval Norse
culture. On the surface of it, the "Hávamál" is the less interesting sort, but it
has a certain appeal: the proverbs quoted in there give a vivid picture of the concerns and
values held by the people of that time. "Cattle die, kinsmen die / the self must also die; /
I know one thing which never dies: / the reputation of each dead man." Hospitality,
reputation, wariness and valor -- it's snapshots of a world very different from the one we
live in today. For writers, the beliefs expressed in the "Hávamál" might well be
more useful than the stories.
But the stories are good, too. The "Þrymskviða" recounts a myth some of you may know about, namely, the cross-dressing adventures of Thor and Loki as they try to regain Thor's stolen hammer Mjölnir. Others recount the stories of mortal heroes instead of gods; the Poetic Edda contains a fragmented version of the Volsungasaga or Nibelungenlied, out of which Wagner made his famous opera. Unfortunately, a gap in the manuscript of the Codex Regius means there is a "Great Lacuna" that deprives of us of the full poetic version.
So there are two reasons for writers and readers of fantasy to pay attention to this text: for the story material therein, and for the general sense of Norse culture. You see a lot of the latter in our genre, but usually watered down into a generic pablum; reading original texts can be a good antidote to that kind of vagueness and colorlessness. This material is also of interest historically, since it was one of the great influences on the founder of modern fantasy, J.R.R. Tolkien. (In fact, there's an interpolation in the "Völuspá" called the "Dvergatal," the "Catalogue of Dwarfs," which has some names you'd probably recognize. Durin? Dvalin? Dáin? It gets worse. Bifur, Bofur, and Bombur, all in series, followed by Nóri, and then later on Tháin and Thorin, Fíli and Kíli. The version in the Prose Edda has Gandalf and a few others, too. Yes, Gandalf was a dwarf.)

If you're interested in reading the Poetic Edda, you have a number of translations to
choose from. I'm not familiar with them all, but I can give a quick overview of a few.
Lee Hollander's is probably the most accurate, in terms of preserving the alliteration and stanza construction that are the hallmarks of eddic poetry; he'll give you the best sense of what it would be like to read the poems in Old Norse. (I'm qualified to make that judgement, as I've done it.) The downside is that his translation can also be very stiff. In order to preserve that alliteration and stanza construction, he sometimes has to go for very archaic and/or obscure English words, so the end result is occasionally obscure. More accessible is Patricia Terry's version, under the title Poems of the Elder Edda; she's possibly too casual in places, but that may be the better choice for readers new to Norse literature. (And as a bonus, she includes the eddic-style poem "The Waking of Angantyr" from Hervarar saga, which is a personal favorite.) Finally, if you want the material without having to struggle through the poetry, The Norse Myths by Kevin Crossley-Holland recounts much of it in narrativized form.
Plus, if this is your kind of thing, a special bonus feature: the musical group Sequentia has done an album called Edda: Myths from Medieval Iceland, which sets to music many of these poems, in the original language. It's odd stuff; I don't know enough to say whether they're accurately representing period music, but it definitely has that sound, rather than a modern one. But I figure there will be at least a few geeks out there who, like me, find such things appealing.

I have a particular fondness for Norse literature, and for the culture that gave rise to
it. They lived fairly simple lives compared with southern Europe, and so their poetry has a
plain-spoken quality that charms in its own way. The humor is simple and blunt -- one of my
favorite poems is "The Flyting of Loki," wherein the gods are standing around having an
argument with Loki; it might be quickly summed up as "your mom!" -- and it does heroism well.
Remember always that the Norse believed the apocalypse would come, and the gods would lose.
But their fatalism is not of an emo, mopey sort; it's cheerfully bloody-minded, accepting the
inevitability of doom and death and then getting on with the business of living. As a result,
the literature carries a raw strength I get on better with than with subtler, more complex
texts.