Now, before we get started, this has nothing to do with the M. Night Shyamalan movie. I know! I leave glasses of water around the house all the time too because of that one, but for right now, let's stay on topic.
Here is another great resource of really random things that those looking to write fantasy, or fiction, or really anything that might need a reference to religion of mysticism might find useful.
The Illustrated Signs & Symbols Sourcebook: An A to Z Compendium of Over 1000 Designs by Adele Nozedar (phew, that's a long title), is another good bedside book to keep handy.
If you're anything like me, religion and symbolism play a role in your work, even when you're not looking for it. The random moment that requires a particular design to it. The layout of a room that might have nefarious purposes later in the work. This is where symbols come in handy. By and far, this is not a complete list. There are undoubtedly more than 1000 symbols and so forth out there in the wide world. Don't be discouraged! This is a great starter to work with!
A lot of the books I'll keep on my bookshelf are resources from a basic point of view. Quick sources that can lead you to deeper questions if and when they come up. Remember, it's important to build your library so that when those questions come up, you have a place to start looking for answers.
The Sourcebook, is a great place to start in that regard! (And it's usually on sale at B&N - I can personally attest to seeing three writers each grab a copy of this book and stand in line one after the other to pay for it on a lark because it called to necessity in their writing lives sometime down the line...and yes, okay, fine, I was one of those darn authors...lay off me...)