The Essential Movements of Tai Chi by John Kotsias is an old book I bought in 1989.
It is not about form. Instead it provides a number of interesting exercises to develop tai chi qualities, such as rooting, springy spine, springy step etc.
There is a good presentation of tai chi principles, such as keeping the weight of the boy continuously going downward, even if the arms or legs are going outward. I like the translation of the Nine Secret Messages by Ng Man Hap at the end of the book such as " the art of confused circles is most difficult to comprehend. However, its changes and variations are mysteriously unlimited".
You can still get second hand copies on Amazon, I believe.