In this company, employees are expected to do their work by the book at all times.

by 在这里表示"依据、按照"的意思。 the 是定冠词，特指某事物。 book 本义是"书"，在这里引申为"规章制度、操作手册"。 by the book 是一个固定短语，意思是"严格按照规章制度办事"，强调做事一丝不苟、不偏离既定规则。