If you want a coffee table book with high quality reproduction of Renaissance masterpieces...buy this book. If you want to know "the secret language" of the "hidden symbolism of Italian Art", then you don't need it. Simply ask the nearest Catholic priest or go to the Catholic Encyclopedia website. The author gives precisely the same information...so how "secret" can it be?
Essentially this is an appreciation of art from the perspective of a dyed in-the-wool Vaticanite. It is fine if you accept the notion that Leonardo, Boticelli, Raphael, Crivelli, etc were pious Catholics who rigorously adhered to traditional Church doctrine. But it seems to me that they were free-thinkers. Doubters were classed as heretics and were not free to openly express their views. That explains why there are so many symbols in these works of art..not because they affirm Church dogma but they deny it.
To give an example, the author mentions that a swallow was a common feature in religious paintings. Swallows migrate every year, nobody knew where they went, and then they suddenly returned. Therefore, the swallow makes an excellent symbol of the resurrection, because Jesus disappeared after his crucifixion, before miraculously re-appearing. It's insulting, isn't it? How about the meaning, widely understood, behind Aesop's fable, "one swallow does not a summer make"? Beware of false assumptions! Things are not what they seem. The inclusion of a solitary swallow into a piece of religious art suggests the artist's true intent is to cast aspersions on common creeds not to fortify them.