Books dating back that far would be extremely valuable as a historical resource. If your great-grandfather is correct, I cannot overemphasize how precious they would be to any historian of Korea. Contact a museum or other institute immediately -- I doubt trying to preserve it on your own initiative would ever be productive.

Still, I would like to see if the books do actually look like something from 10th-century Korea. Actual books actually from that century (as opposed to later copies, which are still rare) are extremely rare throughout the world -- and you say you have seven! Consequentially, I can't easily envision a scenario where an archaeologist would not give such valuable books to a university or museum for preservation and intensive research, but keep it in private possession to the point that it is actively deteriorating.

Holy cats. I agree with /u/PangeranDipanagara -- if these books are legitimately that old, you're probably better off not trying to preserve them on your own. Libraries and museums with the means to house ancient book collections control temperature and humidity to slow the rate of deterioration, and the scholars who study them take special precautions.

I would try to contact an institution about this fairly soon. If your great-grandfather dug them up during the Japanese imperial era, then he must have been professionally active at some point between 1910 and 1945, and the books themselves may have been out of the ground for more than a hundred years at this point. If they've made it this long, that's all to the good, but every day you delay is another one in which they can be lost to fire, theft, or somebody knocking a Red Bull onto them. I've never been in a position to sell or donate an ancient book and can't advise you on what to do, but this information page by the Smithsonian might help a bit.

My first suggestions would be the National Library of Korea or National Museum of Korea, or secondarily Seoul National University. The Koreans have long had problems trying to retrieve books and artifacts that were taken out of the country during the late Joseon dynasty and colonial era, and just from an ethical perspective, I think you would be right to offer the books to Korean institutions first. If you're elsewhere, there are also Korea/East Asia collections in a number of Western facilities (Yale, Harvard, Cornell, the British Library, Cambridge, National Library of Australia, etc.). Most of them are heavily weighted toward post-1800 content, but I think all have collections of ancient Korean texts and the means to house them.

Also during the Japanese imperial times, I'm not expert on this, but handing a Korean historical artifact to Japanese authorities probably wouldn't have survived.

Possibly, but I think their more likely fate would have been to end up vanishing into a private collection in Japan and no one would ever have found out about them. This is a depressingly common practice in colonized or otherwise subjugated states. Although, to their credit, the Japanese have made an effort to find and return some artifacts and texts to the Koreans.

Hiya everyone, OP will post pictures when and if they decide to. Posting "Pics please!" or anything similar will not help, other than to clutter up the thread and thus make our workload more significant. Thanks!

https://imgur.com/gallery/qUw8D

Here is a picture of the random page in the Koryo era book.

If anyone can translate it that would be awesome!

Any update on this?

I know it is late comment, but I still recommend to send the book to library or academia even if it is not from 990 CE. As Korea lost many artifacts to time, warfare and colonialism, the book you have would be still be precious to many academics.