When you take a leaf out of someone’s book, what you are doing is copying or imitating the individual. You are using him as a model and are following his example hoping that you will gain something by this.
Example: “I took a leaf out of Surendran’s book and started submitting my assignments on time”.
The word ‘leaf’ here refers to a page from a book. Therefore, when you take a leaf from someone’s book, you are copying what the individual has written. The original meaning of this idiom was therefore to ‘plagiarise’. Nowadays, the expression has lost its negative connotation and is used only in a positive sense: to imitate someone.
Source: “The Hindu” – Know Your English Column – Jan 22, 2008