What is the diffference between a passport book and a passport card? The only difference that I can find on the Department of State website is that with only a card, one would be unable to fly to Canada and Mexico.

As the other two answers have hinted at, unless you live near the US border or you're traveling by sea vessel, the passport card is rubbish at its stated purpose. However, I still have a passport card anyway, I use it as my primary form of ID, and I even travel with it overseas in addition to my passport. Here's why: 1) It's perfectly valid as identification in the US, and it doesn't have your address on it the way driver's licenses do. I've used it to get into bars, cash checks, and get through airport security. And actually, you can use the passport card to get on a domestic flight. You might have to, soon, if you don't have a REAL ID compliant driver's license. 2) It's a good backup in case you lose your passport while traveling. 3) It works pretty well as ID in Europe as well. They're more used to national identity cards and they see that it has a photo and "U.S. Department of State" on the front, and a machine readable zone on the back. You can't actually use it to enter Europe, but besides that they seem to know what it is and that it's perfectly valid. 4) It's unknown enough that it makes a fun conversation starter.

As already noted, a passport card is a special purpose item, it can only be used for a few limited land and sea crossings.  If you are interested in rapid entry into the US and can qualify, however, it is now far better to have a Global Entry card than a Sentri card.  Global Entry can not only be used everywhere Sentri can to enter the US, it can also be used at a large (and increasing) number of airports. About the only advantage that Sentri has over Global Entry is that it can speed entry into Canada and Global Entry cannot.  But even that can be overcome if you pay the Canadian government an extra fee ($50, I think) to validate your Global Entry card for that purpose.