It is held by a bonus boss in the Alram Highlands. After going through the mines there's a path north that leads to a save circle. Beating the boss will get you the book.

Does anyone know where part three is?

i got books 1-4 but i dont remember what to do next and substory has no details in the menu

honestly i cant remember, i just started going to every possible nook and cranny in the areas i could go to and looking, i grinded up levels so i could beat the scion bosses hidden across the map to make sure i got all the books because i couldnt remember how many books there were total.

 there are four books to find, when you have all four books you speak to mandrake (carrot in the box in the alley) in sylva city, because there isnt a side quest log yet im sharing this. 

There really should be a clue as to where they might be. The Church one is easy, the one the Drug guy holds is easy, the spend 100k at Charlotte's is hidden, but you will get there eventually while fishing the gacha game. The Highland boss though.... its not hinted at, there is not description in the quests as of revision 1.0h, so you don't have a clue where to look. I'm making a guide for this...