The idiomatic expression "close the book" can have a couple of different meanings, depending on how it is used. In most cases, however, this phrase is used to mean "put an end to" or "seal something away." For example, a person may say he has decided to close the book on a relationship when he has decided to end the relationship. He may also use this phrase when he wants to seal a subject in secrecy. In such a case, the phrase often means that the parties involved will not share the information with anyone else; they may not even discuss it amongst themselves once they have agreed to close the book on the topic.
A person can use a variety of different words and phrases to convey the same meaning. The fact that a person has many ways to express the same thought helps to keep language interesting. One of the phrases a person might use to avoid having his speech sound boring or monotonous is "close the book." For instance, a person may want a more colorful way of saying "end" or "stop" and use "close the book" instead.
In many cases, the phrase is used in relation to something the speaker feels is unpleasant. For instance, a person may decide to close the book on a troubled relationship. Likewise, a person might use this phrase in relation to an unpleasant period in his life. For instance, if a person has spent several years struggling financially and then finally has more money and fewer bills, he may state that he is closing the book on the past unpleasantness as he moves toward a more promising future.
An individual might also use the phrase in reference to something he wants to keep secret. For instance, if a person shares a troubling matter with another person, he may decide that he wants to resolve the matter and then ensure that no one else finds out about it. In such a case, "closing the book" may mean putting the matter to rest and then keeping it a secret indefinitely.
It is important to note that deciphering idiomatic expressions can prove difficult for some people. Often, such phrases are well known in one region yet virtually unheard of in another. As such, an individual who lives in an area in which the term isn't commonly used — or a person who isn't a native English speaker — might become confused and believe it literally means closing a book.