Ja(a) in my native language Luo, means a person from a particular tribe / clan, in Swahili it means full.
Pun - A joke exploiting the different possible meanings of a word or the fact that there are words that sound like but have different meanings.
Put together, they mean this book is full puns from beginning to end, also highlighting the fact that I am from the tribe of punsters.
This book is nothing but a Total Joke.