Bryan Lowe, Chris Mayo, and a bunch of other advisors and contributors have launched an online Guide to Shōsōin Research, hosted by Vanderbilt University:

What are Shōsōin documents?

Shōsōin 正倉院 is the name of an eighth-century storehouse located on the grounds of the temple Tōdai-ji 東大寺. While it is most famous for its collection of treasures, including many objects that reached Japan via the silk road, it also preserves over 10,000 hand-written documents all dating from the Nara period (710–784). [...]

Why haven't I heard more about these sources?

English-language scholarship has barely scratched the surface of this rich source base. The primary reason for this neglect stems from the complexity of the collection, which has rendered it nearly impossible to use without specialized training. The manuscripts were cut apart and reassembled multiple times both in the eighth century and in the modern era. Publication further complicated matters, as the documents were again rearranged for the compilation of the Dai Nihon komonjo 大日本古文書 series.

There's also a blog and a glossary, both chiefly focused (at present) on how scholars deal with this cut-up mess of invaluable documentation: "The primary task of Shōsōin scholars in Japan is to figure out how these numerous fragments were once related to one another."

Lowe, Mayo, and the other contributs deserve a lot of credit for their work here. I didn't know diddly squat about Shōsōin documents before reading this page, but I do know how tough it can be to find the first foothold in a field like this — that one obscure reference book, tucked away only in a few major libraries, that lets you start exploring a topic in detail rather than in a popularized (or summarized-for-specialists-in-other-fields) form.