
Richard Williams, notable for his work on "The Thief and the Cobbler", intimately outpours all his animation knowledge to You. The book covers all 12 principals of animation as well as walk cycles, workflows, and general "Do's" and "Don'ts". Although colleagues have told me before that Williams can go a bit too detailed when it comes to information about walk cycles and frame rates.
My initial career idea was to become an animator but I found forums of senior animators complaining about low wages and long hours and it made me second guess myself about all that and I slowly picked up programming instead, I did get a copy of a great book called "The Animator's Survival Kit" by Richard Williams, best known for directing "Who Framed Roger Rabbit?", the book still lives in my library and I hold it in great steem.
[1] https://www.amazon.com/Animators-Survival-Kit-Principles-Cla...
As to how much poor quality in-betweening hurts the performance to the audience is a complicated discussion. Animation that is _very_ bad can often be well accepted if other factors compensate (voice acting, design, direction, etc.)
A good in-betweener is not simply interpolating between the keys. For hand drawn animation at least, there's a lot more going on than that.
We'll leave out any discussion of breakdowns here. For one it's a difficult concept, much more difficult than 'tweening to explain. The other is that different animators will give different opinions on what a breakdown is or does.
I will say, though, I think that properly tagged breakdown drawings could significantly improve the performance of ai generated in-betweens.
Anyone who is seriously interested in the process should read the late, great Richard William's book, _The Animator's Survival Kit_. This is especially true for those who want to "augment" the process with machine learning. The book is very readable, even for non-artists. And he gets into the nitty gritty of what makes a good performance, and the mechanics behind it.
Edit: Another good resource, and relevant to 3D animation as well, is Raf Anzovin's _Just To Do Something Bad_ blog. He has many posts on what he calls "ephemeral rigging" that are absolutely fascinating. Be aware that the information is diffused through out the blog and not presented in a form for teaching. His opinions are fairly controversial in the field. But I think he is onto something. (https://www.justtodosomethingbad.com/)
He spent decades on creating his own 2D animated film called The Thief and the Cobbler which was beset by endless problems and delays. The film was never completed in the form that Williams wanted. The fascinating story of this film can be found in this 2016 article:
13 Facts About The Thief and the Cobbler
http://mentalfloss.com/article/74031/13-beautiful-facts-abou...
Another interesting fact: Williams worked as an animator on the 1968 film Yellow Submarine featuring the voices of The Beatles. I think you can see some of the visual influence from that film (such as the exaggerated forms) in The Thief and the Cobbler too.
Which is also an excellent (very expensive) video series. The videos are probably even more useful than the book.
https://www.amazon.com/Animators-Survival-Kit-Richard-Willia...
Most of the points from the article seem to be derived from the stuff the book covers in more details.