Police Procedure & Investigation: A Guide for Writers. 2007 (Writers Digest Books) is a good place to start. Any recent general how-to-write-mysteries books is likely to have a bibliography for continued research. Nonfiction books by cops are good. (Blue Blood by Edward Conlon, for instance--smart guy, a Harvard grad and former NYPD detective)
There are many books available on firearms, poisons, scene-of-crime forensics, autopsies, and other topics. Your version of mystery writing will require at least some of these. Most best-selling mystery writers have a vast library of reference material and subscribe to professional police journals, too.
It's also valuable to call your local police department and ask to do a ride-along with a patrol car. You'll hear the radio chatter and get a good sense of what working cops do in a typical shift. (They'll talk with fewer curse words around you and put on something of an act, but you'll see some real behavior, too.) If you can develop a friendship (buy the person a lunch/bring donuts to the police station/etc) and have an on-call resource in the police department, even better. You can also sit through a criminal trial at your local courthouse.