In Maplestory, what exactly do mastery books do?

Im a level 100 crusader and I have a mastery book "Brandish", it says if brandish is over lvl 15, I can use the mastery book at a 50% rate to make brandish level 30. But the max lvl for brandish is 20... how is this possible (what exactly does the book do?), how can I use it and what happens if the book fails?