Full disclosure: I'm good friends with author Gene Doucette's mother, so I may not be the most objective reviewer of his works. I did rant long and hard about the all-caps in Hellenic Immortal, however, so I'm not a complete patsy.
Yuletide Immortal is a short story (though it shows up as ~1,300 pages in my Kindle reader; I have no idea how long it actually is, but it sure isn't that long). It is a continuation of Doucette's Immortal trilogy, a marvelous set of stories about (duh) an immortal man sometimes known as Adam. He's not invincible, but he's immune to disease. He's often drunk, usually unreliable, and always entertaining.
The stories have magical creatures (elves, vampires, et al.), but no magic. They take place at various eras in history and involve Adam's (often drunken) adventures around the world. Adam - known as Stanley in Yuletide - is living in New York City in a hotel room with a private bath and not much else, so he spends most of his time at an Irish bar. It's 1952, I think, and Stanley is maybe 6000 years old, but with the maturity of a mid-20s, developmentally arrested frat boy.
The story features Santa, who turns out to be an imp. Imps are long-lived (but not immortal) and love great stories more than truth. Which is why Santa also spends a lot of time in an Irish bar: he tells stories, and hears stories, and drinks a lot.
Adam/Stanley is perpetually pessimistic (I suppose that happens when everyone you know, ever, dies, and always will) and Santa's an optimistic do-gooder; the clash of personalities is entertaining despite (because of?) its inevitability.
All in all, Yuletide Immortal is a marvelous break from the usually insipid Christmas-themed romances that almost always disappoint me (Jude Deveraux's and Judith McNaught's short stories are a notable exception; Debbie Macomber's annual servings of mush are not).