According to the preface, this book is a carefully researched retelling of surviving stories of the Nordic gods, Odin, Thor, Loki, Freya and others. Where surviving versions of a story differ, artistic license allows for the best of each to be used.
First and foremost, these are Nordic tales. If you are familiar with these characters from Marvel comics or movies, be prepared to relearn a lot of what you thought you knew. This, in fact, was one of my favorite things about reading these stories.
The stories are told in a way that makes chronological sense in that things that happen to each character stays with them in subsequent tales, (there's really only one glaring exception to this) though each one can totally be read on its own as well.
Neil is a great story teller, and surprisingly there are points where I was pulled from the story by a needless repetition of fact - perhaps because he is telling these stories in a style reminiscent of the original sources: I'm not sure. What I can say is that if this had been my first introduction to this author, I wouldn't put him at the top of my list. That doesn't say this was a bad book - it really wasn't - it's just that I have lofty expectations for this author and this book isn't his best work.
As this has sources, I am filled with the urge to find out more. I'm curious as to how close to the sources these tales actually are. So, while I might not recommend this as a first Gaiman novel, I would absolutely recommend it as an engaging introduction to the gods of Nordic legend.