Re: [130426] [エウシュリー] 魔導巧殻 ~闇の月女神は導国で詠う~ + Tokuten + Manual [H-Game]
I'm not sure what triggers it, it could be Seenaru store level or the number of stores you have but later on, they will sell books containing monster combinations. There are 9 volumes of it. I got it while going through...