How do you recommend someone with a background in brand strategy/account management break into a PMM role? What learning materials/resources do you recommend for background and ongoing learning?

There is definitely overlap between brand strategy/account management and product marketing especially because the two roles operate slightly differently at every organization. Brand strategy can translate really well to product positioning, messaging and storytelling while account management can translate well to go-to-market planning and roadmaps. I answered another question above on how to break into the PMM field including a helpful storytelling exercise to build out your PMM oriented resume.