> I wonder if this has to deal with some external factors, like availability of a good free book

The Django documentation is absolutely fantastic[1]. It and PostgreSQL are my personal gold standards for documentation.

[1] At least from my experience playing with it. I have not tried to use Django in anger, but it feels like the docs will hold up well.

My only criticism of the Django docs is that every sentence has equal weight. That makes it difficult for someone starting out to distinguish between things that are absolutely critical and those that just nice to know. However once you are up and running they are great but they do need re-reading for the same reason.