Books with code

- 3D Mesh Processing and Character Animation, R.Mukundan, 2022 Springer

- Character Animation With Direct3D, Carl Granberg, 2009 Course Technology

- Advanced Animation with DirectX, Jim Adams, 2003 Premier Press

- Real-time 3D Character Animation with Visual C++, Nik Lever, 2002 Focal Press

Textbooks

- Physics-Based Animation, Erleben/Sporring/Henriksen/Dohlmann, 2005 Charles River

- Computer Animation: Algorithms and Techniques, Rick Parent, 2002 Academic Press

There's lots of open source too

What is a character animation system? Is it something like AAlib?

https://en.wikipedia.org/wiki/AAlib