I haven't read the book either, so I shouldn't comment, but I'm going to anyway.

From what I understand, the premise is that if someone isn't calling you, making an effort to spend time with you, etc. then chances are that they're just not that interested in and you should cut your losses and move on instead of running through the litany of "Maybe he's justs." (You know ... maybe he's just lost my phone number ... maybe he's just really busy right now ... maybe he's just tired and stressed from work ... )
I used to spend a lot of emotional energy agonzing when a guy didn't call after a date. What if my machine wasn't working? What if he'd lost my phone number? What if he called when I was out with my friends and I missed him - would he think I was blowing him off? Should I stay home in case he called? What if he was waiting for
me to make the next move? Should
I call
him instead of waiting for him to call me? If so, what should I say? It was exhausting.

Like I said, I haven't read the book, but just reading
about the book has allowed me to say, instead of agonizing, "Hmmm ... he's not calling. Oh well. He knows where to find me if he wants to." Not that I'm not disappointed when it happens, but it's a lot easier to get over it.
What clinched it for me was when a married girlfriend told me that every guy she'd ever dated had at some point behaved as if he wasn't that into her ...
except her husband. I think most of my married girlfriends would say the same thing. My best friend got married recently, and I was able to observe that courtship pretty closely, and it was definitely different from any she'd ever had before. There was no ambiguous period, ever. No mixed messages. No sitting by the phone or worrying about when she was going to see him again. Not that they got engaged on the first date or anything, but it was clear from almost the beginning that he was most definitely into her, and it stayed has clear right through their wedding day and beyond.
Edit: The book is about men's behavior, but I wonder ... does it work the same on both sides?