Merlin is Robert Nye's irreverent, raunchy and often lyrical (Nye's a poet) take on the Arthur legend. Many scenes reminded me of John Boorman's "Excalibur." To take one example: Arthur and Mordred's fight at their last battle:
Mordred in black armour rode to kill the king. King Arthur ran at Mordred with his spear so that the spear went right through Mordred's body and out the other side.
"Father! My father!" Mordred cries. He thrusts himself forwards along the spear that is killing him. He drags himself on. He crawls slowly, hanging by his wound. He hauls himself inch by inch to reach the king (p. 210).
If you've seen Boorman's film, that should bring to mind the very similar point where Arthur and Mordred meet in battle:
Which is not say that the book (first published in 1978) and the movie (1981) are at all alike. They are very different versions of the Once and Future King's reign.
I came across this book while perusing Merlin's page on Wikipedia. If you prefer your Arthur legends more respectful in tone and execution (and less sexually explicit, e.g., "Sir Gawain and the Sleeve Job"), then this book is not for you. However, I found it enjoyable and would recommend it.