I have several clients in multiple states. We have one master handbook that is considered the master. Then we have state-specific amendments to the handbook for each state that the employer has employees.
State employment laws and requirements change periodically and so it is critical to have someone who is on top of these changes to ensure that not only the handbooks are current but the employment practices are compliant. For example, minimum wage changes can occur in a particular state and if not on top it an employer can find themselves with hefty fines.
If you have more questions, I suggest consulting with an experienced HR professional.