What books do you recommend about religious symbolism?
This is a great question. Let me give you some suggestions that will help you navigate through this important topic.
If you want to look a primary document, which draws on lots of religious symbolism, then look no further than the gospel of John. This will give you a sense of the symbolism that the early church used. Also any commentary on John, such as Raymond Brown's Gospel of John will have an extensive bibliography.
If you want some modern authors in the area of symbolism, start with one of the greatest theorists on this topic. Mary Douglas, who wrote the book called, Natural Symbols: Explorations in Cosmology. The bibliography will be a goldmine.
The gold standard on this topic (apart from Mary Douglas perhaps) is Mircea Eliade. His book, Symbolism, the Sacred and the Arts is a great place to start as well. The key wil be to work through the bibliography and then you will have more sources than you can imagine.
If you want to look at Christian symbolism, look at George Ferguson's Sign and Symbols in Christian Art. It is a bit older, but it will prove to be useful.