what is the "vile booty" mean?
“Aghh, books,” said Cap’n Bones. He spit on the sand again. “I hate books! Keep looking, dogs! Find me something good!”
Cap’n Bones grabbed Jack’s backpack. “What’s in here?” he said.
“Nothing—” Jack quickly opened the pack. “Just paper, a pencil, a book.”
“Another book!” roared Cap’n Bones. “That’s vile booty!”

Question

what is the "vile booty"  mean?
“Aghh, books,” said Cap’n Bones. He spit on the sand again. “I hate books! Keep looking, dogs! Find me something good!”
Cap’n Bones grabbed Jack’s backpack. “What’s in here?” he said.
“Nothing—” Jack quickly opened the pack. “Just paper, a pencil, a book.”
“Another book!” roared Cap’n Bones. “That’s vile booty!”

Answer

'Booty' here is a term pirates used for 'treasure' or 'the gains earned from pillaging and taking what they wanted'.

So 'vile booty' would mean (hated/disgusting/awful) treasure.  The previous sentence has told you the pirate hates books.