First of all, formally, Bu is 4th tone in written Chinese. No matter how you pronounce it, it is 4th tone when you write or type the word in Pin Yin. English has the similar rule, for example, "speak", the written pronunciation is [spik], although we pronounce it [sbik]. Well, now we are talking about the rule of the pronunciation of Bu. Only when the following the word is at 4th tone, the tone of Bu would be changed into the 2nd tone. Example: 1. Bu (2) Cuo (4) 不错（not bad） 2. Bu (4) Hao (3) 不好（not good） 3. Bu (4) Hui (2) Jia (1) 不回家（not go home） 4. Bu (4) Shu (1) Fu (0) 不舒服（uncomfortable）

Your book is right,the right pinyin of 不错 is bùcuò in writting and typing（As youkoufiona said）But in oral we usually pronunce it as búcuò (sometimes even we do not know why,may because so many people say that ,so it becomes a normal usage)

The rules changes for pronunciation rely on you to know them. A book may not change bu to 2nd tone, besides if you type the pinyin, you wou will still have to type bu4, not bu2.

You are right. Your book is wrong. "not bad" should be written as búcuò. Book also will make some mistakes.