When a person uses the phrase "in my book," he usually means from his perspective. For example, a person may say that a flavor of ice cream is the best in his book. This means that in his opinion, that particular flavor is the best. The phrase "in my book" is an idiom, which means it is used figuratively rather than literally. In regions in which this idiom is commonly used, most people understand what it means immediately upon hearing it.
An individual could use any manner of ways to indicate his opinion about something. He could, for example, simply begin his statement with the words "in my opinion." He could also start his statement by saying the words "I think." Often, however, a person uses the phrase "in my book" to make his point. It essentially means the person who is speaking is expressing something from his own unique perspective. He is stating his opinion rather than speaking about something that is actually in a physical book.
The phrase "in my book" and phrases that are used in similar ways are called idioms. An idiom is a common phrase that is used to figuratively express a thought, feeling or fact. Unlike other phrases, they are not meant to be translated literally. This can prove confusing for a person who lives in a region in which the idiom is not commonly used. When a person lives in a region in which it is commonly used, however, he will typically understand what the phrase means without a lot of thought or analysis.
If a person is unfamiliar with this idiom, he may find an example or two helpful for deciphering its meaning. A person may consider an example in which one party is asked his opinion of another person's character. In answering the question, a person may say something like, "In my book, he is reliable and trustworthy." This means the opinion of the speaker is that the subject of the phrase is both reliable and trustworthy.
Another example of the use of this phrase is a situation in which a person is asked to give his opinion on the best places to visit while on vacation. When he gives his opinion, he may state that he has heard of a few places that are good for vacationing, but then say something along the lines of, "But in my book, Grand Cayman is the best place for a relaxing yet fun vacation."