Try Jung's "Mysterium Coniunctionis". It's among my favorite books of his, though I couldn't make it to the end because it triggered feelings that were too strong. It's about the symbolism in alchemy and what it stands for in psychological terms. He goes first through the main symbols, then through the whole transformation process that is supposed to turn other elements into gold, and explains how it corresponds with the inner transformation the adept will be going through (or rather, how the chemical transformation, illusory as it is, actually brings about the psychological one, because the symbols used stir up the collective unconscious). I thought it was fascinating.