As we all know शास्त्र is not a single book, authored by one or more persons, like in other religions. Our Hindu शास्त्रs are Shruti and derivatives thereof, Shruti being अपौरुषेय not authored at all! The Vedas are said to be the breath of Ishvara in Brihadaranyaka by अस्य महतः भूतस्य निश्वसितम्। Therefore, our शास्त्रs are the very breath of Paramatma; and needless to say, no one authors their own breath. This means that they are शब्दब्रह्म and therefore, incorruptible.
To start with, शास्त्रs being available to us is itself शास्त्रानुग्रह। This can also be seen when शास्त्र or its teaching is accessible at the right time. In the simplest experience for many of us, for example, sometimes, you're trying to find an answer to a lingering Vedanta question and the book opens at the exact page holding the answer, etc. One may call it coincidence, but one who has श्रद्धा in how कर्म works, there are no random coincidences.
प्रामाण्यबुद्धि towards the शास्त्र can also be seen threefold as शास्त्रानुग्रह along with ईश्वरानुग्रह-
one, in knowing that the शास्त्रs have एकवाक्यता, that the ultimate subject matter of all the शास्त्रs is जीवैश्वरैक्य, the अद्वैतज्ञान that I am ब्रह्म। To know that this is the single goal of Vedas, among an overwhelming variety of rituals involving मन्त्रs, यन्त्रs andतन्त्रs, involving so many deities representing a single तत्त्व, or several उपासनs with their own फल is not easy to see.
two, ज्ञानेन एव मोक्षः and and three, that there is no other प्रमाण for this ज्ञान। Moreover, we are used to thinking that my experience built across lives gathering data from all my senses goes against it. Naishkarmyasiddhi says: नायं शब्दः कुतो यस्माद्रूपं पश्यामि चक्षुषा। इति यद्वत्तथैवायं विरोधोऽक्षजवाक्ययोः॥ नै.सि. ३.८४॥ “This is not sound”. Why? “Because I see colour with my eyes”. Similar is the conflict between what is conveyed by perception and Shruti.
Even after one has all of the above, still with the same गुरु teaching, one doesn't understand or understands differently as Bhashyakara glosses on the Kena mantra. So there is need for शास्त्रानुग्रह along with ईश्वर-and गुरु-अनुग्रह so that Shruti reveals the right knowledge to us. Why else would juggernauts, who studied the same शास्त्रs, in the same संप्रदाय, who had great पाण्डित्य, who were great भक्तs tapping ईश्वरानुग्रह, go out and start their own philosophies?!
That leads us to the closing question: is शास्त्रानुग्रह really different from any other अनुग्रह? Of course, there is some overlap with others, but the most important अनुग्रह of शास्त्र is मुक्ति itself. In Vivarana tradition, it is accepted that the श्रुति in the form of महावाक्य, on its own, causes अपरोक्षज्ञान The ज्ञानवृत्ति, for which the subject is महावाक्य, destroys अज्ञान thereby resulting in मोक्ष।
Shruti, with Her grace, answers a really unanswerable question in all possible ways, using various प्रक्रियाs, examples and तर्कs so that at least one of them makes the knowledge perfectly clear, even calling herself as मिथ्या so that we don't end up holding on to something that is not real.
May we all be blessed by शास्त्रानुग्रह।
ॐ तत् सत्।