The books, alongside monster hunting books, had been disabled adventurer items. In the past, they had produced five scrolls. I re-enabled them for this Invasion, to give the players another option in combating daimons. I remember writing a comment to reduce the scrolls produced to one scroll when I re-enabled them, but it slipped my mind when I actually got around to doing the re-enabling. When I noticed that, I corrected it back to one scroll as it was never intended to produce five scrolls in the first place. The five scrolls produced after they were re-enabled is the bug, not the one scroll produced, which is the fix. They are not meant to be the sole, effective tool, but one tool in an arsenal. Consider the period of five scroll production a brief divine blessing. Sometimes bugs hurt you, sometimes they help you.
EDIT: After discussion with fellow devs, we have decided to base the number of scrolls produced upon the spellcasting skill of the user, ranging between 1-3 scrolls produced.