April's recommendation: Beowulf.
Welcome to the first of what I guess I could call my folklore recommendations (as that sounds more interesting than "primary source recommendations"). The idea behind this is that fantasy as a commercial genre has very deep roots in folklore, mythology, legend, fairy tale, what have you, and this is something which pleases me; but what does not please me is that many fantasy writers appear to only know that source material second- or third-hand. They're writing stories which were inspired by Terry Brooks who was inspired by J.R.R. Tolkien who was inspired by the heroic literature of the North, and the result is a watered-down, insipid copy which lacks the vital energy of the original. I'm reasonably well-read in various folkloric sources (being a Ph.D. student in the subject helps), so I thought I'd intersperse my fantasy novel recommendations with these.
And, of course, like every good old-fashioned English class, I'm going to begin with Beowulf.
For those of you who somehow missed reading it in high school, or who read it but forgot it, or who maybe are young enough that you haven't gotten there yet (hey, someday I might have a site visitor like that), we'll start out by recapping what it is.
Beowulf is a lengthy poem written in Old English (not to be confused with Shakespeare's English, which many people erroneously refer to as "Old"), in the alliterative style common to the poetic literature of northern Europe. The exact structure of the alliteration depends on the flavor of poetry you're talking about, but in general, the idea is that the lines are not organized according to a rhyme scheme; instead, the alliteration of the words within the lines is what holds them together.
You can think of the poem as essentially biographic. It's about a Geat warrior, Beowulf, and the monsters he kills. No, really. A complex plot is not one of the poem's features. Beowulf comes to the aid of a Danish king, Hrothgar, who's got himself a little monster problem. He's built a great mead-hall, but night after night a creature called Grendel comes and kills his men. Beowulf, being a badass, rips off Grendel's arm, and Grendel goes back to his fen to sulk and die. Unfortunately for the guys back at the mead-hall, Grendel had a mother, and she's worse than he was. Just when they think they're safe, she attacks them. Beowulf chases her into the fen, kills her with a special sword he finds in her lair, and is a hero twice over. Later in his life he fights a dragon, and although he defeats it, he dies right after.
But there's more to it than the simple events. I could do worse than to quote the cover copy off Seamus Heaney's translation: "The poem is about encountering the monstrous, defeating it, and then having to live on in the exhausted aftermath." It's a fine example of what often gets called "northern heroic literature," the literature which so inspired our good friend Tolkien (who, not coincidentally, wrote an essay that had a profound effect on Beowulf literary criticism). Courage in the face of horrific enemies, status through generosity to others, a desire for immortality through great and honorable deeds -- these are the kinds of themes that permeate this sort of literature. There's a grandness to the language, and at the same time an earthy dignity, or at least that's the sense its translators seem to have tried to preserve.
I've only read two translations of it, personally. Of the two, I'd probably have to recommend Seamus Heaney's. It's a facing translation, for starters, with the original text on the left page and the modern English on the right; even when I can't read the original, I have a soft spot for that approach. It's got a good introduction, and the language is accessible. I occasionally have quibbles with Heaney's word choices, but overall, he's a poet in his own right, and it shows. He also provides margin notes that help orient you as to what's happening, and (in a brilliant move) italicizes the two passages in the poem where someone start telling a lengthy side story, which does wonders for helping the reader not be horribly confused about why we're suddenly talking about an entirely different set of people.
The other translation I read was Burton Raffel's; I cannot entirely vouch for its quality, as I don't have the leisure right now to go back and re-read it. Like Heaney's (but unlike, say, the Dover Thrift Edition), it's a poetic translation; it seems to aim more for the dignity and less for the earthiness than Heaney's does. It has a nice introduction and afterword, but lacks some of the other tricks Heaney uses to keep the reader from getting lost. No doubt there are other translations out there, but personally, I have not read any of them. The one I'm looking forward to is the translation that Tolkien apparently wrote but never published; last I heard, it was for some reason being delayed in publication, but hopefully we'll have that one someday.
Unsurprisingly, various authors have decided to have fun reworking this story in modern fiction. Two I'm aware of, but have not read, are Parke Godwin's The Tower of Beowulf and Tom Holt's humorous Who's Afraid of Beowulf? (disclaimer: having not read it, I'm not sure how much, if at all, it's actually based on Beowulf). There are probably others. The only one I have read, at least that I can think of at present, is Michael Crichton's The Eaters of the Dead, which puts an interesting spin on it. Taking as his launching point the historical incident of the Arab writer ibn Fadlan meeting Norsemen on the Volga River, Crichton has ibn Fadlan journey north with them and observe (and participate in) a series of battles against a group of monstrous creatures. It's an essentially non-magical retelling that I have a peculiar fondness for -- a fondness which extends to the movie adaptation, The 13th Warrior.
Speaking of movies, the only other one I'm aware of is a terrible SF-ish adaptation starring Christopher Lambert. Fortunately for the world, we have not one but two new versions coming up, either or both of which will hopefully be a better treatment than that piece of crap. One, Beowulf & Grendel, recently finished filming in Iceland; the other is currently being written by Roger Avary and, yes, Neil Gaiman. My heart melted into a little puddle of happy hopeful goo when Gaiman described it as "a sort of Dark Ages Trainspotting, filled with mead and blood and madness." When those movies come out, I may come back here and update this with my opinions of them.