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
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.
I think 2D animation will eventually make a widespread comeback - ironically probably via 3D animation tools. These tools can now convincingly mimic 2D animation (see Blender's Grease Pencil feature as an example). These tools might not use the traditional frame-by-frame technique of 2D animation, but they still open up lots of possibilities. An example is Spider-Man: Into the Spider-Verse - it's clearly a 3D film but has a unique 2D/3D aesthetic.
I really thought he had the health and energy to finish one more big project. It was clear that he was still drawing and animating frequently even well into his 80s.