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 is a very large book.
Role in the Episode
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 finds his "head" and puts it back on, but to their surpirse it was not his real head. He had become incredibly intelligent. As the title of the book already says, the book is about the physics of having fun.
It is unlikely that this book actually contains knowledge of the physical science of fun, but is actually just a clever title about what fun is