Joe Friel's "Cyclists Training Bible" (book) is a excellent guide to training, but it views HRMs (as well as power meters and perceived extertion) simply as tools to measure your physical abilities along the way to your performance goals. I found this interesting as opposed to books by Sally Edwards (just google) which are very "heart-rate-monitor centric".
Bear in mind that I'm just another interested cycler (i.e., not an expert), but "targets" that may concern you are 1) resting heart rate (as in when you first wake up) - a general sign of fitness, and 2) Lactate Threshold (LT) - point at which your muscles create more lactic acid than can be dispersed; This is important because it can be lowered through training so that you can, for example, climb longer and farther before you "burn up". LT is usually close to the rate between which you are aerobic and anaerobic. Point is that you can express these levels of exertion as a percentage of max heart rate. If you watch the pro cycling races on TV, they now sometimes post the rider's current heart rate (at least they did on the Giro this year) - very interesting to see some guy whizzing along at 30+ mph and his HR is 120!
I bought an HRM (actually I bought several, but that's a long story) and wear it regularly on my rides - not necessarily as a training device. It's quite interesting to see what's going on as you climb hills, cruise, etc.
Try googling "cycling" and "HRM", "LT".