@srowdy after reading your comment, I think the time period is somewhat off if you said it was during the Qin Dynasty (cause that would be 2000ish years off) and it was clear he serve during the Second World War as a code breaker, and Xin Fu, for his case could have die and was brought to that world. As for the MC, either he die a Konosuba way or like the common Isekai style was drag into that world.
For the LOTR book, I would guess that the skeleton was a US or Western Pilot during the Korean War who was also brought into this world while (a wild guess here) reading that book and since Xin Fu was a code breaker, he should understand English and after reading the book, named the people of that world form what he was able to understand the description from the book. And of course for what happened to said book, depending on the location he was at, the book could have easily rotten away if the place was humid enough.
@goodhunter and
@hullmat991 the MC could simply be drag into that world like some old Isekai and time in our (in a sense) world could have slow down while he is at that world. As for the other Human, well same thing, either they were able to die or was drag into that world