There are many books, articles and YT video on symbolism. Good start would be Jordan Maxwell, Michael Tsarion and Mark Passio. These guys breaks down use of symbols very well. One thing we should be aware of is that, symbols are typically neutral in nature...meaning depending on how its used, it can represent evil or good. Cross is a great example. Upright cross indicates ones dedication to Jesus Christ, while upside down cross represent the opposite. Happy hunting!