This section only includes her Arthurian works as I think that they are the best and the top of the range in the works. As I have read Heaps and Heaps of Arthurian works and almost everyone has included a statement like "The Mary Stewart of tomorrow" type quotes. So if you are interested in the field read these works as they are VERY good and they don't contain Lancelot (yay!)
The Crystal Cave - This novel starts of the Merlin series with stuff from his past like his mother and it just is a new perspective on Merlin's boyhood and him starting to grow up - I'm sure the creators of the Merlin miniseries would've referred to this work in order to start it off. She deals with the usual but writes them in such as away that you feel like you haven't heard the story before.
The Hollow Hills - Continues on the story and deals with a lit of Arthur and his court like Guenevere and her treacherous love with one of Arthur's knights and she isn't dealt with heaps nicely like those Guenevere novels by Rosalind Miles which make you feel like throwing up. And the story of Arthur isn't clichéd and boring and it really deals with Merlin and his 'magical powers.'
The Last Enchantment - Draws the doomed story to the end with Merlin becoming encaved and Arthur dying and the like but it was still compelling right to the end. The ending has a sense of continuation: " I left the night, with its remote and singing stars, and came in, to the glow of the fire, and the chair where he had been sitting, and the unstrung harp." These books are a must read because I can almost guarantee that every fantasy and historical fiction writer has at one time or another read these works.
The Wicked Day - I didn't know this existed for a long time after I had read the Merlin series and I was treated to her writing all over again. As this is a tale of Mordred and how he grew up and became that man that everybody today hates but really the death if Arthur was just one part of the truly Wicked Day. It is compelling and she plays with Mordred in such a way that you don't look at the character in the same light ever again.