A lot of working people don’t have regular free time to acquire new knowledge after leaving the university. A lot of skills can be learned and provided in training, however many “hard” knowledge like mathematics, economics, philosophy are not available to at work, and they require relatively vigorous training. Many still want and need to learn new hard-skills (as opposed to soft skills) after university. For me, I really want to learn more about mathematical analysis, statistics, and maybe information theory. These are useful subjects which I did not master or neglected during my university time. Besides, after working for just few years, I realized subjects like economics (some general knowledge and monetary policy), philosophy, politics are tremendously helpful.
For most “typical” working person, finding regular long hours to sit down and study is really hard. Most people will have 1-2 hours free everyday: these including commute time, evening time (if you are still single), toilet time (applicable for many). With these constraints, the usual textbooks do not work. I have tried before, it was really hard to use the normal textbooks. I think there are few criteria which make a book suitable for a working person:
* Self-contained: The book should provide all the background knowledge to follow the materials in it.
* Written in small pieces: so that people can read in a short time (on bus), gives them some senses of achievement.
* Concise: we don’t have much time.
Of course, the last point is desired for all kind of writings, but the first two are not necessary for normal textbooks.