I was in middle school at the time, learning programming. I think by that time, we had home Internet, but it was really slow, and so a lot of my learning about programming was either from the help files that came with the IDE (at the time for me learning Java, that meant Microsoft Visual J++) from books I convinced my dad to get me from Barnes and Noble as a reward for doing the "important" summer reading. I had a couple Java books before, but they were more focused on the mechanics of the language. Your book was the first that talked about how to put together a system.
As a specific example, I had never been exposed to much in the way of networking concepts before. Black Art has a chapter in adding networking functionality to a game which was my introduction to networking concepts and how to build networking software. Until much later when I learned about higher-level frameworks, I used the design patterns for network programming in several projects in school over the next several years.