I'm writing a mystery novel and have just started a short story that includes the police too. The problem is everything I know about police procedure comes from television or from pulp novels circa 1952.
Luckily, Lee Lofland has written Police Procedure and Investigation: A Guide for Writers. He also has a blog called The Graveyard Shift about police matters and writing, including interviews with published authors, agents, and editors.
Cops 'n' Writers is a consultation service that might also be worthwhile, if I had an advance from a publishing house (and if the cops in my writing were American).
At least the research books and the consultation could be claimed as expenses at tax time.
Writers Write also has an article stuffed with links on internet law enforcement resources. I haven't checked any of the links yet, but even the titles make good starting points.
Fabulous links and a way better layout can be found here at In Reference to Murder. This amazing site is put together by a mystery writer/librarian - check out his/her homepage and blog, too!
Lastly, keep in mind the various yahoo groups and forums you can join to glean info from people. I was part of a gun group on Yahoo for a while that was an offshoot (pun!) of a firearm training school (hello, CSIS!). Although I eventually left 'cause I just didn't need that much information, if I ever did need more info on guns for my writing, I'd go back. The people there have access to a lot of professional, technical, and firsthand info and experience that would take me much longer to amass on my own.
I only know one police officer, and I don't want to wear out my welcome by pestering her with too many questions, so it's good to have places to read up on the basics first. If you have any of your favourite research links to share (especially for genre writing), please let me know in the comments!