iPhone Books app (EPUB files) and iTune music library files storage location

How/where can I copy (1) EPUB book files and (2) iTunes music files from my PC onto my iPhone 6s? A full restore from my iPhone backup only brought some of the music files in, and none of the book files. (The files HAD all been on my phone when I did a full backup of it two weeks ago.)

The Apple store was replacing my iPhone battery, but could not get it to reboot. So they very generously gave me a brand new iPhone 6s that had never been used (not reconditioned or anything).

When I got home with the new 6s, I tried doing a full restore my my 2-week old full backup of the phone. My old apps, bookmarks, notes, photos, settings, etc. came over, but only a small portion of my iTunes music library and NONE of my BOOKS EPUBS and PDF files were restored.

How can I copy them from my PC backup folder to my iPhone? I can copy the files via Windows Explorer, but I don't know where to put them on my iPhone so the Books app will see them.