One of the key markers of intelligence is the ability
to manipulate symbolic thought.
Symbols are found in mythology and folklore worldwide,
and here are some books at this site on this topic.
The Migration of Symbols
by Goblet d'Alviella 
The migration and mutation of a set of powerful archaic symbols.
The Book of Talismans, Amulets and Zodiacal Gems
by William Thomas Pavitt and Kate Pavitt 
The symbolism of gems and their astrological signatures.
Architecture, Mysticism and Myth
by W.R. Lethaby 
The macrocosm reflected in ancient architecture.
Fictitious and Symbolic Creatures in Art
by John Vinycomb 
Lore of the fantastic beasts of English Heraldry.
The Signature of All Things
by Jacob Boehme, translated by Anonymous .
A Christian mystic explains the hidden meanings implicit in everything.
The Secret Teachings of All Ages
by Manley Palmer Hall 
A comprehensive introduction to the Esoteric, including the Qaballa, Tarot, Ceremonial Magic, Neo-Platonic Philosophy, Mystery Religions, Rosicrucianism and Freemasonry.
The Gnostics and Their Remains
by Charles William King 
King's pioneering summary of what was known about the Gnostics prior to the Nag Hammadi discoveries.
Includes extensive discussion of Gnostic symbols and beliefs
among the Masons, Templars, Rosicrucians and Illuminati.
The Hieroglyphics of Horapollo
translated by Alexander Turner Cory 
An attempt in late antiquity to explain Egyptian Hieroglyphs as pure symbols (very unsuccessfully). Noted for its influence on later occultists.