It is a subjective phrase, of course other books may be more important in other contexts, but this book is able to grip a being at their core, and shake them straight as a nail, if understood.
The book - Sacred Speech: The Way of Truth Speak by Tamarack Song
Of its premise is that we are fundamentally set up to lie, fib, and smudge the truth. We do it to avoid hurting others, and we do it to keep ourselves out of danger. Ironically, these two reasons of doing it are actually the end product of our interactions when we do not speak truth.
In short - truth speaking is that way of speaking, that voice and language, that vocabulary, that feel in words that provokes seriousness, and makes us focus on what is being said. We become one with the speaker and take what is said and understand it on a level far beyond that of just diction. We have communicated to each other in the fullest way possible.
I urge all that see this post to read this book. It can be downloaded for free, or purchased: