Pretty sure this is a bug, as I can't recall going 1 day w/o receiving at least 1 ascension skill book a day since they came out. Though I play for many hours daily, largely in sleep mode, not black spirit mode and though I spend the vast majority of this time farming Elric Shrine, a new 'ascension' area, I believe not a single ascension skill book has dropped for me in the past 3 1/2 days, since the last patch.