Who Stole What? (Who Stole from Whom? Part IX)

One day, Mike, Spike, and Slug went to the neighboring town of Middleberg and committed three robberies. One of them stole a rifle, one stole some money, and one stole a book. The three were caught, but it was not known which man stole what. At the trial, they made the following statements:
Mike: Slug stole the book.
Spike: Not so; Slug stole the money.
Slug: Those are both lies. I didn’t steal either!
As it happened, the one who stole the rifle was lying, and the one who stole the book was telling the truth.
Who stole what?

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

Mike cannot have stolen the book, because he would have said that he stole the book. Slug cannot have stolen the book for the same reason. Therefore, Spike stole the book, which means that Slug stole the money and Mike stole the rifle.