The Police Manual was used by the Officer to note differences in animals.
When the Officer was about to take away Rudy and Slimer for having an illegal monkey. Rudy asked if the officer had something to distinguish monkey from a ghost. He pulled out the Police Manual. The manual noted that monkeys are brown, have a tail, and eat bananas. A ghost is noted in that book for being green, eating anything, and are extremely slimy.
Then Rudy pointed to the Organ Grinder that was seen with a monkey which was seen pick pocketing someone. The Organ Grinder tried to convince the officer he has a dog. However, the monkey actually takes the officers had book out and points out he is a monkey.