This is an Arthurian trilogy by Bernard Cornwell. The three books are The Winter King, Enemy of God and Excalibur.
I came across this book when I was reading about the recently released movie King Arthur (yeah, the one with Keira Knightley in a leather bikini playing Guinevere). Apparently no one is quite sure if King Arthur was a real person on whom the adventures are based or if he is just fiction. The earliest Arthurian tales come from the Welsh. They speak of a great warrior who fought the invading Saxons so well that the saxon invasion was temporarily stopped.
The story of Arthur and his brave warriors adopted by the bards during the middle ages. This is where we see concepts like The Round Table, Arthur-Guinevere-Lancelot love triangle, Knights embarking on quests, knights displaying chivalry(a concept by which you seek to defend women against all men but yourself), knights following a code of honour even unto death, knights being good christians and all other spiffy stuff.
I am given to understand that the womenfolk of the early middle ages pretty much paid for such stories to be made up in the hope that the menfolk, who were brutal knights who bathed maybe a couple of times in their lives, and who spent time swinging huge implements capable of causing much blunt trauma at each other and pretty much everyone else who came in their way, would take up these ideals. Ok, I am being more than a little facetious :-)
Suffice it to say that most of the stories of King Arthur that we know today are latter-day additions by French and English authors and were written to appeal to a medieval sensibility.
Bernard Cornwell takes a different tack. He sets the tale in the early sixth century AD. The Roman empire has withdrawn from Britain, leaving behind a group of states which are constantly jockeying for power. The saxons (a germanic people) are invading from mainland Europe but the Britons (the present-day Welsh) are not in any way united enough to put up a common resistance.
The tale is told from the perspective of Derfel Cadarn, who is now a monk in a monastery but was once a warrior. He had fought alongside Arthur until the end. Derfel, the narrator, is a son of a Saxon slave who is brought up by Merlin. The story, revolves around him and we get to see his childhood, his training and his battles. At the beginning of the book, Derfel is an old monk and he has a constant visitor, the queen of the realm who wants to know all about Arthur and his wonderful band of warriors. Derfel, having been one of them, tells her the stories. So, we see the entire tale in flash-back.
The Arthur in this book is a warrior and an able leader of his men. His one great weakness is an irrational belief in the goodness of other people. All he wants is a little farmhouse where he can hang out with Guinevere and raise cattle. Alas for him, he is too good a warrior not to attract attention and the people around him a bit too jealous and ambitious. He comes across as a good man, who tries his best to keep his promises.
He lives in troubled times, however. Merlin is trying to put together the thirteen treasures of Britain so that he can summon the Old Gods of Britain. Every king and chieftain is trying to figure out how to defeat the others. All the while, two Saxon chieftains, Aelle and Cedric are trying to take over Britain, whilst simultaneously fighting each other. All the usual characters from Arthurian tales are present, Arthur, Guinevere, Merlin, Galahad , Lancelot etc. There are, however, no women in lakes distributing swords nor does Merlin conjure up thunderbolts.
What we have instead is a fascinating description of how the Britons lived. We get a glimpse into the politics and religions of the time. The battles are very well written with vivid descriptions of the tactics and the action. I rather like the sword-and-shield-adventure genre but this book rises above that. The story told in the book covers nearly 60 years but one never gets the feeling that the book is slow. As an aside, since the book covers such a large time-span, the author lets us see how the legend is already starting to develop. So, what was to Derfel a near defeat has now been sung into song by bards and now the people believe that it was a briliant victory. Of course, Derfel knows what the real deal was with the Round-Table, but no one wants to listen to him, least of all the queen. People want the glamour and 'spin' has a long history :-)
The author has done a good job of incorporating the Arthurian tales into a semi-historical setting. One can see that a lot of work has gone into historical research of late fifth century Britain.
In all, while it is not exactly Lord of the Rings, it is a great read if you are looking for a nice adventure.