This is the sequel to the Colour of Magic, and the second book in the Discworld series. Now, the Discworld novels are not strictly sequential and while it is not necessary to read them in order, because the books tend to be grouped into the main characters (such as the Wyrd Sisters, Death, and Rincewind) it is helpful to read the earlier books before the later books.
I preferred this book to the Colour of Magic, one of the reasons being that it actually had a plot. The first book simply has Rincewind, Twoflower, and the Luggage, stumbling from incident to incident and finish up by falling off the edge of the Discworld. This book begins where the last one ended (and the 'heroes' are saved, but I won't say how), and it turns out that Discworld is heading towards a star and the closer to the star Discworld gets, the weaker the magic gets. It is believed that the only way to save Discworld is to cast the eight spells of the Octavian, however the problem is that one of them happens to be trapped in Rincewind's head so he suddenly discovers that he is the centre of attention.
Rincewind is the classic 'unlikely hero' in that there is nothing about him that stands out - in fact he is a bumbling fool not to mention a complete coward who spends more time cowering under tables and running away. However, despite bumbling everywhere, he manages to actually succeed (though more through luck than any skill). He does have some companions, one of them being Cohen the Barbarian. True to Pratchett's style his barbarian is not the thumping muscular brute that Conan is, but rather a toothless old man with a club. However, while he may be past his prime, he is still a formidable foe, and still has sex appeal.
The other interesting character is the Luggage. It is what its name says it is: a large chest, with lots of feet, and a pretty psychotic personality (okay, the name doesn't suggest feet, or a bad case of psychosis, but it has that nonetheless). It is more like a pet than an object, though one should not get on its wrong side. In fact, it does play some important roles in the books, and in the end is gifted to Rincewind when Twoflower decides to return home.
All in all I enjoyed this book, though unfortunately you have to read the first one first to understand this one. At least it has a plot.