What are the most prominent Tibetan Buddhist texts that are only written in Tibetan script? A lot of Buddhist texts come from Sanskrit or Chinese. I am looking at the Kangyur and Tengyur, and a few scattered others. But a lot of those are from Sanskrit and Chinese as well (or other languages, Pali, etc.). What books or groups of books/works can only be found in Tibetan script? Whether or not it has been translated to other languages isn't important, the original language should be Tibetan though.
I am trying to mentally gather an image of how much reading would be required to gain the knowledge unique to Tibetan Buddhism.
The foundational tantra (book) of Tibetan Dzogchen , the Dra Thalgyur, (“The Reverberation of Sound”) is only available in Tibetan. If you are interested in the highest practice in Tibetan Buddhism, that would be one you would need to read.