Beyond the official Jinja docs, there’s the Jinja for Ninjas tutorial. However, it is a bit old… so more recently added functions are not included. We also have a number of Template related articles in the Cookbook.
The most up-to-date source for HA-specific template info are the HA Templating docs.
HA’s implementation of Jinja also allows a number of basic Python functions, but those are considered outside the scope of the Home Assistant documentation.