There's a lot of collaborating going on in the mystery genre. Whether it is bringing together several characters from different series to solve a crime, or authors working together on a novel, collaboration brings an interesting perspective to the story.
Some mystery authors who write more than one series will occasionally bring their characters together on a case. J.A. Jance united her Seattle detective J.P. Beaumont and Arizona sheriff Joanna Brady in Partners in Crime. Robert Parker has included both P.I. Sunny Randall and police chief Jesse Stone in Blue Screen and High Profile.