I never read the Prydain books by Lloyd Alexander as a child because the library didn't have them. I knew about them from reference works on fantasy and children's literature, but I lived in a small town and I don't recall having access to interlibrary loan. I was reminded of them by Alexandria Digital Literature, and I was able to track down some copies at the local used bookstore.
I read these at the same time as Susan Cooper's "The Dark is Rising," alternating between the two. This, of course, has linked the two series in my mind, and I find myself forced to compare and contrast the two series. This is somewhat unfortunate, because two authors and two series can hardly be less alike. Yet they are both for young adults and both contain installments that won the coveted Newberry Award.
The biggest difference between the two is that Alexander's style is much more readable, much less "literary." If I was to put reading levels on the two series, I would probably set Prydain at 5 and Coooper's books at 7. Although both book series are fantasies, "The Dark is Rising" is much more realistic. Prydain has a Tolkien-ish feel to it, while Cooper's world resembles Mary Stewart. Alexander goes for humor, whereas Cooper often goes for suspense.
The first book, The Book of Three, introduces Taran, the assistant pig-keeper, an orphan who longs to be a hero. He gathers companions, some honor, and a little humility in accomplishing his goal.
In The Black Cauldron, he gathers his companions together for a special mission to capture the cauldron from the diabolical Arawn.
The third book has him moving into late adolescence, where he must take his friend and, as he realizes within this book, love, Princess Eilonwy to The Castle of Llyr, where she must face her origins.
The quest in the fourth book, Taran Wanderer, is to find out about his own origins.
The final book is the award-winning The High King, and it finishes off the series with a grand climatic battle between the forces of good and the forces of Arawn.
Reading them all in a fairly short period of time, I noticed the "updates" that Alexander uses for those readers who may be coming to each book as their first or after a longer period of time. They're short enough that a judicious editor could excise those sections and combine all the books into one hefty novel. Between Alexander and Cooper, I prefer Alexander, but I suspect that it is as much for his humor than for any quality difference.