This book, published in 1938, gives some historical information on an Egyptian pharaoh called Akhenaten, who was the first (and only) Egyptian pharaoh to pursue an exclusive monothestic religion, meaning he only worshipped one god and insisted no other gods were worshipped. Being that he was pharaoh, the whole of Egypt had to comply. His reign didn't last that long.
Freud then starts theorising about the origins of Judaism based on what is known about it from historical evidence and biblical teachings. His views are highly controversial and have been dismissed by many; however it does provide some explanation on, for example, why the Jewish Old Testament God is so much different to the New Testament Christian one.
Worth a read, in my opinion.