Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Meditative, Thoughtful, Brooding
It's not a surprise that Stewart's prose feels a bit emo, especially with a tragic character narrating the entire story from memory in old age. The best is clearly in the past for Merlin, and a life full of war and following "the god" has clearly taken its toll. Check it out:
I am an old man now, but then I was already past my prime when Arthur was crowned King. The years since then seem to me now more dim and faded than the earlier years, as if my life were a growing tree which burst to flower and leaf with him, and now has nothing more to do than yellow to the grave. (Prologue.1)
Of course, the pace picks up as the story gains momentum, but don't expect that melancholy and thoughtfulness to go away. If only Camlach hadn't tried to poison our hero with a tasty apricot, or if only Keridwen hadn't rejected him...