March's recommendation: Monstrous Regiment, by Terry Pratchett.
One day, when I was not feeling well and had finished the book I'd gotten out from the library, I was casting about trying to find myself a book not set in the nineteenth century (which took more tries than you might expect), and picked up Monstrous Regiment. Which I got as a freebie at World Fantasy, I think, but hadn't yet read because it's something like the hundred and twelfth Discworld book, and I've only read a scattered four or five of the preceding hundred and eleven.
But I picked it up anyway -- and oh, am I glad I did.
First things first: you don't need to have read everything that comes before. Or even anything that comes before. Sam Vimes is a recurring character in the series, I know, but his part in the story is minor. Anything you need to know about the setting, this book gives you, and there's no feeling that you're missing out by not knowing more context. (If you are missing anything thereby, then Pratchett's done a beautiful job of catering to both new and experienced audiences.)
So anyway, the book. It is delightful. Pratchett really has grown out of the early days of his humorous fantasy and into something that still makes me laugh out loud on a regular basis, but has a great deal of substance underlying the funny. The point here is not jokes; the point is satire about army life, and the reasons why people fight wars. Borogravia is at war with Zlobenia -- well, they've been at war with anybody close enough to attack for quite some time. But this time Zlobenia's brought friends to the fight, and although supposedly they say Borogravia is winning, you'd be hard-pressed to find anybody who's actually saying that. Most of Borogravia's able-bodied men have already enlisted, gone off to war, and come back missing body parts if they came back at all.
Although Vimes appears in the story early on, I think it's mostly so he won't seem to come out of nowhere later on. The main character is Polly Perks, a young woman who disguises herself as a man and signs up for the army in order to search for her brother Paul, who enlisted a while ago. It's a setup we've seen before in fantasy, but this is Pratchett, and that means he touches on the usual tropes only insofar as they serve as stepping-stones to the things he really wants to be doing. Polly's tiny group of recruits ends up being nicknamed the Monstrous Regiment, because Borogravia's really scraping the bottom of the barrel: they've got a troll, a vampire, and an Igor, a person who does various odd kinds of doctoring and body-part transplant. They're under the command of the infamous Sergeant Jackrum, a larger-than-life figure who's every motif of sergeant-hood blown up to epic proportions (including size). And, despite the fact that they say Borogravia is winning the war, this sorry-ass band of recruits gets no equipment, no training, and no orders but to head straight for the front -- where there may or may not be an army left to join.
Much beyond that, and I'd be in spoiler territory. Suffice to say that I had quite a fondness for pretty much every member of the Monstrous Regiment by the end, though I think Maladict might be my favorite, a vampire under an oath not to drink blood: "Good evening, gentlemen! Please pay attention. I am a reformed vampire, which is to say, I am a bundle of suppressed instincts held together with spit and coffee."
Odds are that many of you reading this know about Discworld anyway, and don't need my recommendation to point you at it. Some of you, though, may have stayed away because you don't think humorous fantasy is your cup of tea. My advice is to try this one, and if you like it, find someone who's read the whole series, and get them to tell you where to start among the later books. They're funny, but they have a purpose in life beyond being funny; the ironic moments and entertaining turns of phrase serve the greater cause of the story.