By now, I've had a few questions of the form Why didn't you cover [topic X] in TLPI? Examples of topics that have been mentioned include more socket options in Chapter 61, tracing and debugging techniques, and POSIX AIO.
The general answer to this question is twofold: time and space. There are various additional topics I myself would have liked to include in TLPI, but in the interest of actually finishing the book, I had to draw the line somewhere, and exclude those topics. In addition, as we headed toward production, it became clear that we were going to run close to the limits of what could be printed as a single paper volume (the publisher and I both agreed that we didn't want to go to two volumes). Thus, there wasn't enough space to cover every desirable topic.
Assuming that there is one day a second edition of TLPI, we may well go to two volumes and go into greater detail on some topics and add various topics not covered in the first edition. I'm collecting suggestions for topics to consider for inclusion in future editions of TLPI, if you want to send me specific suggestions.