I think the best course of action for information on local and/or state law enforcement is to go directly to the source. As Patrickl said, many departments have brochures you can read. Also, many agencies have a Community Policing or some sort of public relations department that can point you in the right direction. A ride-along could give you an opportunity to speak with an officer, and a chance to observe what he does during a shift. For federal law enforcement, there's always the Internet (of course). Hope this helps....
The first thought that popped into my head was, check your local university or junior college for law enforcement classes or degree programs. If they have such, then check out the college bookstore to see if the books for the LE classes have the info you are wanting.
But that might not get you what you are seeking, if you are mostly interested in a reference on agencies and departments, what their qualifications are, what they pay, etc.
If God didn't want us to own guns, why did He make the 1911?