Dear Keith,
Some people swear by plumb bobbinbg, and others swear it doesn't work. I'm sort of in the middle, because I think it sort of works to confirm your intuitive read on some putts, doesn't work at all on big breaking putts, and regardless of reading the slope can give a helpful "true vertical" reference. My take on all this is in the article "Plumb Bobbing is Plumb Crazy -- Mostly" at
http://puttingzone.com/MyTips/plumb.html.
Here are some people who write about plumb bobbing -- its effectiveness and procedures:
Larry Stanley's The Putting Edge,
http://www.theputtingedge.com/
Roger Griffin - UK PGA - Putting Tip - How to Plumb Bob Your Way to Better Putting
http://www.pga.info/experttip.asp?id=201
Ken Tannar - PGI - Reading Putts & Plumb Bobbing
http://probablegolfinstruction.com/id18.htm
Golf Tips | Become An Expert Green Reader | Plumb Bobbing Technique, By Andy Walters, PE
http://www.golftipsmag.com/content/pastissues/2001/jun/greenreader.html
Bogart Golf - Swing Library - Plumb Bob
http://www.bogartgolf.com/lesson/swings.asp
Golf Digest: Plumb-bob to confirm your first impression. Issue: Jan, 1999 BY JUSTIN LEONARD Winner: Players Championship
http://www.findarticles.com/cf_0/m0HFI/1_50/54390594/p1/article.jhtml?term=putting
Dave Pelz - The Golf Channel - Plumb bobbing
http://www.thegolfchannel.com/core.asp?page=32100&select=72
Phil Richards - Secrets to Plumb Bobbing
http://www.golfsupport.com/richardstip.htm
Cheers!
Geoff Mangum
Putting Theorist and Instructor
The PuttingZone
http://puttingzone.com
Golf's most advanced and comprehensive putting instruction.