I own Game Engine Architecture 2nd Edition and it covers skeletal animation but I find it too hard. Can anyone recommend good sources on this subject?

Was just about to recommend Game Engine Architecture. That together with googling resources is all you need.
Frank. D Luna's book is also ok, but i wouldn't buy it just for that chapter.

EDIT: Some useful links:
http://mmmovania.blogspot.se/2012/11/skeletal-animation-and-gpu-skinning.html

http://gamedevs.org/uploads/skinned-mesh-and-character-animation-with-directx9.pdf

https://www.opengl.org/discussion_boards/showthread.php/179419-Need-help-with-skeletal-animation

https://www.gamedev.net/resources/_/technical/graphics-programming-and-theory/skinned-mesh-animation-using-matrices-r3577

https://www.gamedev.net/resources/_/technical/directx-and-xna/animating-characters-with-directx-r3632