Mythology and divination are great topics to brush up on. Also symbolism and herbalism. Symbols are a historical thing, so most libraries have books on the subject. Understanding the history of a symbol can aid you in using it in your craft. And herbalism/natural medicine is a great way extend your magick work in herbs.
Check if they have anything under astrology as well. Learning about the planets and constellations through astrology can give you a far better understanding of correspondence times and ingredients if you follow the almanacs (I personally do not). Not to mention, understanding the planet's personalities can help you to better understand what it means when a book says that, for instance, that white sage is under the planet jupiter.
Philosophy! Did you know that the elements we use -- Air, Fire, Water, and Wind -- originated from Aristotle?
Psychology! Jung was extremely influenced by the occult. There are many psychologists that are.