I have 17 Mystery Mastery Book's from a long time ago (first time playing maple in ~9 months)
And every time I try and open one it says 'Use item inventory is full.'
I have tried to open them on multiple characters across 5 accounts and none of my characters can open them. Is there an npc I have to see to open them or should I just sell them in fm to some unlucky smuck >.>
They might not be working just because they're old. As new classes are released, new mystery mastery books are released that add their skillbooks to the list. I'm not sure if that's actually the case, but that's my theory anyway.