There is one final book in Shi'ari grove offered by the shop, you have to choose to not leave for T'yani grove yet when you arrive there before the end sequence. Hope this helped!
I don't know, I don't find it. I "Restart(ed) from T'yani".
I already bought every book in H'yatis's Store. I select "The War Band" > "Wait, I need to do something first" but no new book appeared in their store.
This is not a book I would say I found "somewhere in the wasteland".
Interesting, I have this achievement and only ever remember getting books by purchasing them from the various settlements. It's possible I found one by complete chance somewhere and didn't notice having an extra but I wouldn't be able to help you there
