The exit plan is first and foremost. If its only been 3 weeks in between, its been going on for a long time.
Read up on the cycle of violence. It documents the subtle signs and behaviors that occur each cycle. The timing within the cycle shortens as it continues. Information on anger management - for both parties is good for awareness. Almost all domestic violence websites have an exit button to quickly hide any searches. My post Look Through the Lens of Her Dark Sunglasses and See The Strength - has some additional resources.