The last book contains a secret. What appears to be gibberish is actually a set of acrostics where the first letters of each word spell out the cryptic directions to finding three [[Secrets#Locked_boxes|Secret Statues]] located around Pelican Town:
You will receive a different secret statue as a reward for depositing each required item into its respective locked box hidden throughout town. For example, placing [[Duck Mayonnaise]] into a hidden purple box in the back of the [[The Stardrop Saloon|Saloon]] will give you the secret item [[??Pinky Lemon??]].