As your question didn't include a reference to a specific book, the best I can do for you is help you to understand tone a little better.
An author's tone is the overall emotional charge of the writing they are expressing. Typically when evaluating tone, begin with the simplest dichotomy: is it happy or sad? Once you've determined whether it is positive or negative, it is possible to qualify it further. Is the writing serious, or sarcastic? Humorous or morose? Determining a satirical tone can often be challenging, as you have to pick up on very subtle details to recognize the sarcasm or satire of some authors (i.e. Jonathan Swift).
The best indicator of tone is through word choice. Why does the author choose one word over another? If you notice a pattern as to why various words are chosen, you've most likely stumbled upon the author's tone.