I'm a player in a Deathwatch game, and I'm enjoying it. A few years back, I watched a demo of Dark Heresy, and it looked fun, but I just didn't get into the game at that time. Now that I'm looking into the other 40K RPGs, there is a daunting list of Dark Heresy books on the shelves at the FLGS.
What I guess I'm wondering is the following: If I'm interested in the core rules, Sisters of Battle, and interacting with Deathwatch at Deathwatch levels, which books should I look into, and is there anything else that is so essential to how the game have developed, it's silly for someone new to overlook that book?