I've read this book through several times over, but I carry it with me on my daily commute so that I can go back and read another chapter if it's been a while.
This book has been out there for some time, but its advice still applies today. This is my best example of how his advice helped:
I was arguing with a bank branch manager who was telling me that the check I was depositing was going to be subject to special holds. At the time, not being able to use the money wasn't critical, but it was inconvenient, and damned annoying since I knew there wasn't any problem with the check. I had raised a points regarding the source of funding, rules on clearing, and level of customer service I was receiving. The manager responded with several points, including, how this was for my protection. My annoyance was heading into anger when I remembered Dale Carnegie's advice to avoid getting into an argument. Once you're in a argument, he said, you're not going to win, no matter how effectively you press your point. I hadn't avoided the argument, but I could still keep from making things worse. I took the check back, and then stopped at the branch that was close to my home (the first branch was close to where I work). There, the branch manager politely asked about the nature of this check. I told him, and things proceeded normally.
This book provides Dale Carnegie's advice and techniques for interacting with people, but, even more than that, it invites you to adopt an outlook and understanding that people do things based on their beliefs of their valid needs. If you respect that when you're dealing people, you're much more likely to have a better, agreeable outcome.