Unique Map won't drop Atlas Tree Book

Reference number: 2,701,786,842

Currently I got 38/40 points of the Atlas tree. I only need 2 more that I can get from the Unique map Silent Cave.

I fought the Essence boss in the Unique Map Silent Cave twice. One time on a T12 map and one time on a T15 map. However, after defeating the boss, the book you get to spent two points in the Atlas tree didn't drop.