The Physics of Fun is a dark chartreuse book with the text written in a faded brown color. The book seems to be very thick, so the assumption can be made that it's a very large book.
Patrick reads this book to find out what has changed since he fell off a cliff in Jellyfish Fields. When he fell off the cliff, his head had come off, so SpongeBob found his head and put it back on, but to their surpirse it wasn't his real head. He had become incredibly intelligent. As the title of the book already says, the book is about physics of having fun.