By the book
Definition 1 of 2
Pro Tip 1/3Adherence to Rules
This phrase emphasizes strict adherence to established procedures, often in bureaucratic or formal contexts. 
The auditor went by the book and found several discrepancies.
Pro Tip 2/3Lack of Flexibility
It implies that a person is not willing to improvise or deviate from established standards, even when a situation might suggest it. 
Despite the unusual circumstances, she insisted on doing everything by the book.
Pro Tip 3/3Negative Connotation
Sometimes implies being overly strict or unimaginative, especially when flexibility could lead to better outcomes. 
His by-the-book approach often frustrates his creative team members.