[tjr]

Often, the raw knowledge regarding programming is just as freely available as the tools: official documentation for compilers, interpreters, libraries, whatever, or, failing that, the tool itself as a source of knowledge.

Many such resources are available free of charge.

Other people come along and write different materials based on that same foundational knowledge. Maybe they are better writers, and can express the same thing in a more understandable way. Maybe they have a different slant on the whole topic, like the difference between reading The Little Schemer and the Revised Report on the Algorithmic Language Scheme.

In any case, these authors certainly may introduce something new: their own writing! If you don't find any value in that over the baseline documentation, then you are not obligated to read it, but I see nothing wrong with authors charging for their own creative work, should they so desire. (There is also nothing wrong with them giving it away for free, should they so desire. It is theirs to distribute how they see fit.)

It is conceivable that the original documentation and/or product could be licensed in such a way that further works describing the same content could be inhibited. I vaguely recall some open source product that was developed and supported by a commercial entity forbidding commercial documentation from including the name of their product in some form (while further non-commercial documentation they found acceptable).