Frenzy Skill Book

What map can I obtain one?
It's on expert maps right

Nowadays (after the v1.2 update), you may only get it on the book chests. Normal-type Book Chests may be found on any Expert rank mode but I did hear that people are seeing less and less of them since the patch. I have had the same experience recently.
Try and think of another advanced skill to go for and farm that. You might see the Normal-type chest while doing so!
It might be extremely hard to get it now, though. You need to be lucky when opening those chests to get the exact skill you want.

Nonetheless, good luck!