This started as a low budget BBC movie and the script got turned into this book.
It's all about the London below, the underworld, where magic is common and people who live there are all but invisible to the people living above.
I love the story and the characters, especially Richard Meyhew. Richard is from the above world and completely over his head when he rescues Door, a princess from London below who desperately needs his help.
If you are trying to pick between the movie and the book, I would pick the book, since the BBC movie REALLY shows its budget.
This is probably my favorite Neil Gaiman book, and there are a few good ones, including "American Gods" and "Anansi Boys", both of which I enjoyed. Of course, his comics are even better, and if you can get a graphic novel version of his Sandman series, do so, you won't regret it.