As far i know, there isn't any, but a good tip is to check the mobs levels according to the skill book level, they usually have a relation between them, when they are close in lvl of each other, then those mobs usually drops them
Skill books mostly drop from bosses and from supers(blue) campion(red) but can to drop off normal mobs and it acording to lvl off moobs so chek advisnig npc in bw to see wer u need to hunt for ur lvl .
bosses that drops noob books that some find a bit hard to find .
Bosses in BD drop some ,Terror tempel boss empor serpent , holy Palace bosses zumadugon bosses ,mutan mine bosses , priaanja icland Caves bosses .
so yust hunt find out what you can solo or kill in grup .
Personally think that some of these books only “realistically” been available through gg is a good thing! Keeps both currencies in a healthy state. This is something that I think Andy should pride himself on as most gms on previous servers I played got greedy at some point and ruined all their hard work