The best way to learn about writing is with practice and practice and more practice. Read books on writing and work with a critique group. Attend writing conferences and mystery conventions. The information found there is invaluable.
Here are links to some mystery conventions and writing conferences:
http://www.mwa-florida.org/sleuthfest.htm
http://www.malicedomestic.org/
http://www.omaha.lib.ne.us/mayhem/
http://bloodywords.com/bw/index.html
http://www.partnersincrimeboise.com/murder.htm
http://www.deadlyink.com/conference.html