During my most recent long hiatus from the Guild (yes, I'm a butthead) I ended up buying this book and it's wonderful:

How to Draw and Paint Fantasy Architecture

Rather than actually being a how-to-draw book, it's a good resource for stylistic elements of many types of historical architecture, along with giving some great advice on how to combine them in different ways for a fantastic yet believable architectural style.

At $12 or so on Amazon, it's well worth the cash. Highly recommended.