Amazon says 12.88 for paperback, new. You can get it used for about 7$.
Ultimately, the benefit you'll get is in putting the recommendations into practice in some fairly consistent way. So, for example, for chapters that suggest changing your behaviors, you should make a commitment to work on them every day, or five times a week, and stick to that for 2-3 months. It is almost exactly like going to the gym, where consistent workouts will get you in shape, while just working out very hard every now and then won't help all that much. I've found that gym analogy to be nearly perfect for SA. Your behavioral experiments are your "social workouts" and not letting yourself skip workouts will bring the best results.
So keeping a record of your achievements, which I believe the book recommends, is helpful. It lets you know you're staying consistent, and lets you feel proud of your progress -- just as a workout log does at the gym.
A great idea in the book is to be on the lookout for whatever opportunities come up to challenge your social anxiety in small ways, then take them. If you start brainstorming, you can usually think of countless small social challenges that you can put in your social "workout log".