I have the first three books, and completed the Radiation Gate. According to the extras guide, the fourth is supposed to be held by a guy in the Daath library that leads to Mt Zaleho. Problem is, there's nobody there. Is there some other activation flag for this, or is something bugged?
According to Cyllya's sidequest guide(http://talesof.cyllya.com/toa/mhm/sidequest.html), the starting point for the last book is simply when Eldrant is available. Though I don't really know if this means when Eldrant first appears, when you get the option to fly to Eldrant, or after you fly to Eldrant.
"It's not working. The banana is not melting." -Masaokis