Light Fantastic by Terry Pratchett

 This is direct continuation from the first book "The Colour Of Magic" which ended pretty ridiculously by everyone falling off the rim of DiscWorld. Well, apparently the spell that got lodged in the head of Rincewind does not want him to die and every time he is close to death, the spell saves him. So, even now, as he is falling into the void, outside DiscWorld, passing by the elephants and The Great A'tuin, the spell teleports him back on the surface.  Somehow, Cohen the Barbarian (82 years old) and Twoflower also find their way back to the service from the "space" vessel that was used by the Krull Empire, so all of them start walking back to Ankh-Morpork. 

On the way the meet Trolls, who apparently are living rocks and cannot be differentiated from regular rocks unless they decide to move or speak. Also, since rocks include precious stones as well, there are all kinds of diamonds, rubies, sapphires, emeralds, etc. to be gathered from a body of a dead troll, which is what happens (through a mediation from a local robber gang) and Cohen finally gets his lost teeth replaced by a denture made of pure diamonds, so he doesn't make the "shhh" sound anymore.

Anyways, the gang discover that magic is disappearing from DiscWorld because A'Tuin is flying (swimming?) towards a great star, and the air is getting hot and rarified. The local mobs create a movement of "Star People" who murder all the wizards and everything magic-related, because they think they are at fault for their world flying into a burning star. 

One of the wizards from the Unseen University steals the Octavo book, which is now pretty harmless, with the magic gone from the world and tries to force Rincewind to surrender the last, eighth spell, so he can rule the world. This leads to an arena duel in the "Dungeon Dimension" which is populated by all kinds of vicious monsters, more terrible than anything seen and all sounding very Lovercraftian. 

Rincewind wins, says the eight spells from the Octavo and magic comes back to the world. Twoflower decides to go back home to the Agathean Empire, having decided that he had enough of touristing, and gifts The Luggage to Rincewind, who first refuses, but eventually accepts it as a companion. 

Overall a good book, though too much emphasis is placed (again) on parodying different Epic Fantasy/Sword and Sorcery tropes, and not that much on development of an interesting plot. In any case, Rincewind is a very likeable character who continues in several other books (together with The Luggage), but it is a shame that Twoflower doesn't appear again (except through his descendants one more time). Many tropes used have been later reused by J.K. Rowling in her Harry Potter series (the Room of Requirement), but Pratchett remains the original.