The Slightly-Chewed Insult Book is an item tied to Pirates' Day, which is on the same day as the International Talk Like A Pirate Day. It is only for 24 hours, so if you want this item, you need to play on this day, or else you need to wait until next year.
In order to get the Slightly-Chewed Insult Book, you need to kill a mob called Ol' Eary. He is a giant shark that spawns at 37 67 The Cape of Stranglethorn, which is just outside of Booty Bay. Anyone who tags him has a chance to get loot, and the dropchance of the book is about 33 %. His respawn timer is between 15 and 30 minutes, so it should be pretty easy to farm the book if you want it, and you can kill him as many times as you will until the book drops. As said above, he will only spawn during the Pirates' Day event, so don't bother looking for him on other days.