I've got a copy of The Complete Training of Horse & Rider by Podhajsky but my favourite gurus are Vladimir Littauer & John Richard Young.
Jean Saint -Fort Paillard explains well the meanings of terms used in equitation & Tom Dorrance, if you can follow his writing- starts the mind working. Dorrance was very much a pioneer in what I think of as the enlightened method of handling horses but his book is made up of a series of anecdotes - it is not a text book as such.
Sadly I notice most of the younger generation do not read books and in truth much of what we are rediscovering has been known for a long time.
However I must confess, that my own very skittish mare, Delta (stable nickname DiDi, is teaching me more practical handling than the books did.
She has definitely taken over from where Joe left off. Only yesterday there was an incident which I must write up soon.