Quite a few more than just 4. Pretty sure I figured out how to get the game to recognize the books. I mentioned getting a data.pak dump from someone else, which contains the books with improper unicode names. The game recognizes those.
Yeah, it'll recognize the filenames as long as they are in whichever codepage your system is currently running.
So if you use Windows in English by default for example, it has to be mangled the same way Korean would be mangled by the English codepage. If you use Windows in Korean by default (or run the game through LocaleEmulator in Korean codepage), they have to be in proper Korean. Etc. It's that kind of situation. That said, I can only find 4 book text files in the latest version from Helly, so I'm not sure where the others have come from! If you have Helly's versions of them then that would be very useful for us to finish up Helly's patch. I know how to get the original files for all of them (as shown in your picture), it's Helly's translation that concerns me.
The game should "switch over" to a higher quality version of the soundtrack while it's running, from 11khz to (apparently) 24khz. At least, it does so for me.
The ADP files in the BGM.pak file are 22,050Hz, but this is still very low quality audio compared to what is common nowadays -- it just doesn't sound very good by modern standards. I'd personally gladly redo all of the music files using the official soundtrack as a source if I had a way to encode IMA APC ADPCM.