I'm sure the first thing that pops in your head is... why are there 5 different books here for a Trilogy?  Depends on how you look at things.  The hardcover (listed first) contains the 3 books listed next. (The Crystal Cave, The Hollow Hills, and The Last Enchantment)  The trilogy follows the life of Myrddin Emrys or Merlin if you prefer.  I love books 1 and 2 and found book 3 of the trilogy dry and boring.  This does boil down to my interest as in books 1 and 2, Merlin is young to middle age.  He's an old codger in the 3rd book.  Book 4 follows Modred, King Arthur's bastard son by incest with his half-sister (per this version of the tale).  This book is also quite interesting as Mordred is not a bad guy in this tale and the conflict with his father the King is not intentional.  

The reason I like the books... The Crystal Cave is the coming of age of Merlin.  The Hollow Hills is the coming of age of Arthur.  And The Wicked Day is the coming of age of Mordred.  Mary Stewart does a great job of coming of age.  The Last Enchantment is about the end of Merlin's days and that's not nearly as interesting to me, at least.