Videos
A youtube playlist that shows how to start making hand drawn cel animations.
Walk cycle examples by Toshiyuki Inoue
Studio Wan - Has some useful tutorials. I especially liked their Retas Studio tutorials.
Dong Chang - An actual anime animator creating tutorials on youtube.
Books
Documents
A google sheets document maintained long ago by some people on sakugabooru. - I found this on sakugabooru IRC channel.
Other Websites
Sakuga Booru - A booru dedicated to sakuga.