It is, of course, a retelling of the Beowulf saga from the point of view of the monster who wrecks Hrothgar's meadhall and feasts on his men.
I love it because it's a fiercely intellectual book, concerned with truth and ultimate meaning. I love it because it has so many fabulous lines. I love it because the dragon Grendel visits is one of the best characters ever created in all of literature.
I love it because plot is never the point: if you've read Beowulf, you know how Grendel ends: Beowulf rips Grendel's arm off, and Grendel goes off to bleed to death in the woods. So you don't read it for what happens, you read it for how it happens, and why what happens matters.
I get annoyed when people refuse to know anything beyond the initial set-up of a book they want to read or a movie they want to watch. "Don't tell me! Don't ruin the end for me!" they shout, covering their ears, as if ignorance is a necessary ingredient for enjoying art. If I feel I'm getting too caught up in wondering what will happen next to appreciate things in a text like musicality of language and construction of scene, I'll read the end so I can just dispense with the suspense and concentrate on enjoying the pages before the end, rather than racing through to the end.
The best books remain compelling and worthwhile even when you know exactly how they end: you enter the world of the book and that world takes over. I've read Pride and Prejudice at least fifteen times, and every time I read it, I am as engrossed, as anxious to read the next scene, as if I didn't know the story at all--because Austen's prose is just so good, her insight into human beings so clear-eyed and astute, her narrative so breathtakingly complex and rewarding. I reread it this summer and had to stay up until 3:30 in the morning to finish it--I just couldn't put it down until I was done.
Grendel is the same way, and I love it for that; I love that its world is so compelling. I also love Grendel for the stark, empty epiphany he has as he confronts his death. He insists even in the final moments of his life that everything is a matter of chance, that nothing is fated, but at the same time, one choice is as good as another. He refuses to believe that Beowulf managed to hurt him through anything but accident, fortunate for Beowulf, unfortunate for Grendel.
I will cling to what is true. "Blind, mindless, mechanical Mere logic of chance." I am weak from loss of blood. No one follows me now. I stumble again and with my one weak arm I cling to the huge twisted roots of an oak. I look down part stars to a terrifying darkness. I seem to recognize the place, but it's impossible. "Accident," I whisper. I will fall. I seem to desire the fall, and though I fight it with all my will I know in advance that I can't win. Standing baffled, quaking with fear, three feet from the edge of a nightmare cliff, I find myself, incredibly, moving toward it. I look down, down, into bottomless blackness, feeling the dark power moving in me like an ocean current, some monster inside me, deep sea wonder, dread night monarch astir in his cave, moving me slowing to my voluntary tumble into death.
In On Becoming a Novelist, Gardner discusses that scene and comments that while writing it, he was thinking "child thoughts of death with undertones of guilt and the ultimate moral ugliness of God." I have always loved both that phrase and that idea. I do think the idea of God, at least in his Christian form, is one of the most morally repugnant ideas humanity has ever invented, in part because God is so capricious--fate is a matter of his choices, in which one choice is as good as another: he can choose to destroy the world by flood, and then choose not to, and it's all pretty much the same as far as morality and ethics go, because he's God and gets to say so. When I still believed in such a creature, I also often felt like I was falling off the world into some endless hideous darkness.
Which maybe is another reason I don't mind knowing how things turn out.
But don't let the fact that I've provided one of the last paragraphs of the book and the idea behind it prevent you from reading Grendel yourself if you haven't already. It's so good! And since you know now (if you didn't already) how it's going to end, take your time and notice how inventive and insightful the book is, and don't worry about the plot.