The Last Enchantment, published in 1979 (the year I was born), deals more with Arthur, Nimue/Niniane, and Merlin's final repose than with Merlin's life. When I first read this book, I hated it. I didn't think it stacked up at all against the other two books, and I wanted Merlin to remain strong and powerful, lively and vibrant. Re-reading the book years later, however, I saw how strong Merlin had remained... and how he still lived through Arthur.
Arthur, the King, has relied on Merlin, his Cousin and first advisor, for too long. Now, he must learn to let go and stand on his own feet.
Nimue/Niniane is the first real female character in the Merlin books (other than a looming female threat, or a vamp, or a mother), which is odd considering that the author is female, with a penchant for strong female characters in the other books she's written. However, the Merlin books are written from Merlin's perspective, and his great downfall is that he really knows nothing of women.