Heather's right on BMI, but it is a useless stat. If you're measuring weight (and your height is staying constant) then you know your BMI by default.
You're thinking body fat percentage and um, hello! was I the only one who took caliper readings in 6th grade gym? I mean, I was at Polaris at the time so things were pretty weird, but I assumed that using calipers was like a standard class in grade school gym. You can get some cheap ones online or like at a sports store. It is literally a way to measure skin folds, which sounds gross, but it is the only cheap, easy, and reasonably accurate way to measure body fat, which seems almost a more important metric than total weight.
As for books, I've been kind of obsessed with a book called The 4-hour Body lately. There is lots of interesting info there, and it is all delivered in a kind of "hey, there is science to support this, I tried it, here are my results, you can try it too if you want, but if not, no big" way. Sugar Blues is an oldie but a goodie. Diet books (I don't subscribe to the diets wholesale, but they are each interesting to think about) I've read and recommend are Abs Diet, South Beach, The Paleo Diet, Primal Blueprint. Skip anything that has a celebrity on it. Workout books are all the same and rarely offer anything new.
I don't even think that reading on the topic changes my habits that much. It is fascinating to me, though, so read I must. Don't even bother buying books; I will go skim through if I've got an hour to kill near a Borders, and most books only have about an hours worth of actual meat to them. The rest is usually schedules or recipes or whatever.
I like a cheat day because it means I can have my cake and eat it too. Literally, but just on Saturday. And since I know on Sunday it is back to normal, it isn't like a relapse. Also, I don't think you can get too mad at yourself for binge weekends on occasion, or even when you're put in social situations from time to time that require you break your own rules. I avoid beer for health/calorie/glycemic reasons, but when someone buys you a drink, you can't just say no. It isn't small things like a donut or a weekend that will make or break you; it is the larger picture and your lifestyle choices as a whole.
