According to the preface, what distinguishes an eternally lasting spiritual text from one that is merely a product of scholarship?

The preface, as detailed in Chapter 0, distinguishes between poetic books that are natural, graceful, or artificial. It argues that a book written solely on the strength of scholarship and intelligence might be good, but it will not last eternally. To have an enduring impact and be trusted by seekers, a spiritual book must possess the qualities of inspiration, grace, and a divine gift. These are qualities that are difficult to attain, regardless of one's scholarship, without the grace of a Guru. The Shri Sai Satcharita is presented as a 'grace-filled' book, placing it in this higher, more enduring category of spiritual literature.