You are probably familiar with Beowulf. We've all read it, parts of it or seen a movie or tv show based on it. We know the basic story even when we HAVEN'T read or seen it. It is like Sherlock Holmes (to use a more recent fictional example). You know who he is even if you've never read a word of the source material.
Gardner's book is completely the views of Grendel. In the old epics monsters were rarely given a point of view. Here he is, sometimes, eloquent (when he isn't disemboweling people or biting off their heads). More important Gardner 's prose is mesmerizing and intricate. One character speaks with modern idiom but there is a reason for this--a reason I will let you figure out for yourself. There are ruminations on the meaning of life, the lack of meaning, the mundane nature of evil and the nature of time itself. This is truly the sort of book you keep thinking about after you are done. You may even pop back and re-read passages you just finished. But keep in mind here this book never "bogs down" or becomes tiresome. You want to know what is on the next page and then the one after that.
It is a short book, under 180 pages in the edition I read (pictured). You can easily finish it in a day or so, even when you backtrack here and there. John Gardner tragically died in a motorcycle accident at the age of 49 and he also wrote a number of academic books on writing as well as children's books. This book inspired me to buy a number of the academic books on writing.