I love this series - adventure, Catholic and world history, corruption and thrills all wrapped up with the Vatican Secret Archives as the kicking off point for each adventure. There are running threads throughout (the dastardly Dante for example) but each one can be read as a standalone, though it's richness comes from the core group of characters who I've come to feel real affection for over the course of what is now 5 books spent with them (an initial trilogy and this follow on series).