Being The Sea of Trolls, The Land of the Silver Apples, and The Islands of the Blessed.
These books actually remind me of Juliet Marillier's in atmosphere, but are somewhat cheerier and quite a bit more action driven. I was actually surprised when I got to the end that Farmer fit so many different episodes into three books, though some of them are just that: episodes that may add a little to the world-building but not so much to the plot or character development. (The chapter "The Sacrificial Stone" comes to mind; creepy, but no further relevance except so far as it reflects the overall theme. It actually stands out because she does such a good job of making other seemingly unimportant details tie in to the plot later.)
I admit, a bit before the ending I thought to myself "This is Norse-inspired, it's not going to end well, is it?" and was (bittersweetly) surprised. (IMPORTANT NOTE, avoid Farmer's blog if you don't like spoilers; she answers a question about what actually happened at the end although I find myself disagreeing with her interpretation.)
So why did I love these books? Honestly, by the end of the first book (which is a pretty good adventure in itself; in fact, all three books could probably stand on their own, but will spoil events from previous books), Thorgil had stolen my heart. The story is nominally about Jack, a young Saxon, who is apprenticed to a Bard who showed up in his village several years before the story begins and started giving orders. As often happens in stories, Things Go Wrong. Jack is kidnapped by Northman berserkers, ends up on a quest, and so on. Along the way he meets Thorgil, a "wannabe berserker" (description straight from the cast of characters) around his age who hates Jack but is also straightforward, loyal and even kind, as long as no one is watching. The wicked sense of humor doesn't hurt, either.
The thing I found most troubling is the statement (and what happens in the plot backs it up) that you get the afterlife you expect. Berserkers end up in Valhalla, druids and heroes to the Islands of the Blessed, Christians in heaven, and so on. There's also a lot of "Christian magic" which, of course, is not much different from any other magic. (The book credits this mostly to Sts. Patrick, Columba and so on who, we're told, were also druids.)
Still, I'd love to see the further adventures of these characters. Whether it will happen is anyone's guess (the third book isn't selling too well, apparently) but even without a continuation these three make up quite a good adventure with deeper themes, history, well-done humor (it relieves tense moments without making them seem frivolous or unimportant) and great characters. There are a few plot holes (the beginning of the second book rather jars with the character development at the end of the first) but it's a lot of fun.