You must find all 3 of Gruntilda's spellbooks for this achievement. Here are there locations and how to get to them:
Book 1. The first book is after BubbleGloop Swamp, level 4. So once you learn how to wear the boots on this level from Bottles just leave and cross the Swamp and turn right into the tube (3 tubes, forward leaves area, left goes to Brentilda and right goes to a pair of boots.) Get the boots on and run through the swamp and behind the enternce to BubbleGloop swamp there is a passage, go through it and once on the other side, go up the hill to the left and break the thing blocking the passage, now return to BubbleGloop Swamp and turn into an alligator. Leave the level as the alligator and go through the tube again and go to the spot where you destroyed the block. Go through there and there is Spellbook 1.
Book 2. You will need to be a pumpkin in order to get to this spellbook. You will become a spellbook on the Mad Monster Mansion level, which is level 7. Once Mumbo turns you into a pumpkin, leave the level and bounce out of the area. When you are in the volcano area there will be two paths, take the skinny one. Once you get across it there will be a small hole, go through that and there it is, spellbook number 2.
Book 3. After you have entered the second code come back to the graveyard. Go to the left of the enterence of Mad Monster Mansion and break the fence. Now go into the level and turn into a pumpkin then leave. Once you leave take a sharp right and go through the fence that you knocked down and go into the hole in the door. Go to Mumbo and turn back into Banjo. (Mumbo is in that shack) Beak Bust the sarcophagus in the center of the room. Now jump on the first water switch.
Later on when you leave Rusty Bucket Bay, enter the alcove holding the grate which you knocked out earlier. This is the room where you first hit the water switch. (Not the one in the graveyard) It is the one with the shrapnel in it. Go to the opposite way to the third water switch. (Go through the pipe) Slam down on this and jump into the water and enter the next room. You have 30 seconds so quickly swim up to the top and enter through the topmost alcove. This is the room with the Rusty Bucket Bay enterance. Inside here, you will find the third and final spellbook.