I've not heard of any companies (except some colleges) funding actual books in a similar way -- probably because if you pay someone to improve Django, your product (based on Django) improves, and you presumably make money from your product.

Paying someone to write general documentation helps train both your and other's developers -- but the link to profit is more indirect -- writing proper text books is not something that makes sense to in-source for most institutions (shorter tutorials, technical documentation etc, being something different).