This book was published by a small press in what was most certainly a limited printing. It does not appear to have been widely distributed but for years was offered by a small mail order book seller specializing in New Age titles.
Interestingly, Dr. O'Neill is a research biologist by trade who is primarily known for his life long work in environmental science with over 200 publications under his belt most of which are in his field. This book, however, appears to be an exception, being well out of the area of his chosen field. In fact, his bio on the Oak Ridge National Laboratory web site does not even see fit to mention his anomalous venture into the Tarot and Renaissance cosmology.
According to the back cover of this book, Dr. O'Neill developed an interest in mysticism during his training for the Catholic priesthood which included several years spent in a religious cloister.
He has a broad knowledge of Western philosophical and mystical traditions that is expressed well in Tarot Symbolism. There are chapters on the Italian Renaissance, Neoplatonism, Gnosticism, mystery religions, Hermetic traditions, heretical Christian sects, Kabbalah, alchemy, numerology, and astrology. He summarizes his own purposes in the opening paragraph of his introduction:
The enigmatic images of the Tarot have fascinated authors for two hundred years. Occult writers have done much to elucidate the meaning of the symbols but have accepted fantastic theories of origin. Authors interested in playing-card history have assembled the documentary evidence needed to reconstruct the true origins of the cards but have largely ignored the symbolism or reduced it to trivia. The present work does not fall neatly into either camp. . . . The major onus of the book is to present the symbolic systems of Renaissance Italy and to suggest how these systems might have entered into the design of the Tarot. The book does not offer a definitive interpretation but presents the available data from which such an interpretation might eventually be constructed.
Dr. O'Neill immediately accepts the now widely held conclusion that the Tarot form of playing cards originated in Renaissance Lombardy. In his opening chapter, he summaries and dismisses various theories of origin such as the Tarot being invented in ancient Egypt and the Tarot being brought to Europe by the Gypsies, etc. Other theories such as the Tarot being invented by or at least influenced by Jewish Kabbalists, or similarly by alchemists, he is less dismissive of.
Through the remainder of his book, Dr. O'Neill explores the various influences (see list of chapters above) as they were known to have existed in the Renaissance. He goes into some detail in each area and relates symbolic aspects of each tradition to the images found in Renaissance Tarot decks. His overriding supposition is that the Tarot are an expression of a "Western mystical tradition with philosophic roots in Neoplatonism and religious roots in Gnosticism and the Mystery Religions." However, Tarot Symbolism has value beyond the author's final conclusions as it is a rich source of comparative symbolism and meaning.