The Animator’s Survival Kit by Richard Williams is an essential book to own whether you’re a beginner or an expert.
Live2D has a bunch of software that allows 2D artists to create animations from their artwork. Available in both FREE and PRO versions.
SloMo Ref is a free collection of slow motion animal reference videos on Youtube
Endless Reference is a Youtube channel that focuses on animation reference. Lots of great human locomotion videos from different angles
Tri Vital has created a collection of awesome Maya rigs and put them on gumroad for free!
Bodies in Motion is an ongoing photographic project that aims to document the power, beauty and possibility of the human body in motion. It is freely accessible to all visitors at preview resolution or you can subscribe for higher quality images.
MakeHuman is a free open-source software that allows you to create and pose 3D human characters. Useful for pose reference.
Splatfrog is a website with hours of lecture/demo videos by veteran animator, Ken Fountain. It has both free and paid videos.
Thinking Animation has a great section on the pros and cons of different rigs. As well as a bunch of other resources and tips across the site.