One of my favorite trilogies-- and my favorite retelling of Arthurian myth. This trilogy by Gillian Bradshaw is sympathetic and with characters understandable to modern readers.
Hawk of May centers on Gwalchmai, a version of Gawain, this time a son of Lot. He's a compelling character, a weak warrior who finds a cause and his destiny. He's the only really high magic character-- everyone else scorns and disbelieves magic... in the daylight. The situation feels authentic, with numerous kingdoms in turmoil, striving and ignoring the looming threat nearby.
His mother, Morgawse, is half a goddess and completely consumed by darkness. Gwalchmai struggles with doubt (his own and others'), but finds a solid path. The relations between the brothers is very well written, and changes surprisingly as the story goes on.
Kingdom of Summer is trickier; while the viewpoint character changes (to Rhys ap Sion), the story focus really remains on Gwalchmai. Shifting the viewpoint allows us to see how extraordinary Gwalchmai is, particularly for his era and profession. Rhys is well motivated and clearly drawn, but he's not the high magic hero of the first book.
In Winter's Shadow was hard to enjoy the first time; I was unsympathetic to Gwynhwyfar and heaped the blame on her. Since she was the viewpoint character, I had a lot of problems enjoying the book. On rereading, I cut her a lot more slack and came to sympathize with her. Each time I fall a little more for her; her struggle is understated and her story starts late-- the great efforts of her early years are short flashbacks, not lived.
Medraut is compelling, though his persuasion has to be chalked up as supernaturally effective and his motivation seems thin. Despite that, he's an excellent foil, one on whom all of the characters can project their own darkness. Bedwyr becomes more contemptible to me, but I understand his pain and need for love.
The end is excellent; while you know everything is failing, you hope that some brightness can be saved and passed on. You're rooting, even when all seems lost.