A book apart knows who they are for.
There are a million books for people who want to learn how to build websites. A book apart is for the people who want to design websites.
A book apart knows what they are for.
It is very easy to get tangled up in all of the standards that exist in the realm of the web. A book apart is for making a brief and powerful statement that will help designers everywhere make better decisions with their sites.
It is easy to approach any project with the mentality of : "Why are people going to want this?". The harder question to answer is: "Where does it fit?"
If you can identify why the problem you are solving is important, you can start to answer that question.