I'm at a company whose PMM team doesn't have a clear vision for how they will grow next year (in terms of headcount). I'm a part of the customer success team, and I'm trying to do different things to prepare to apply for when something does open up. Do you have any suggestions for the kinds of things I should be reading/working on?

I would start by focusing your research internally - although the PMM team might not know what headcount they have yet, I can guarantee that they know where their gaps are / what areas they're struggling to keep up... How's their partner go-to-market strategy? Do they feel they have strong joint value props and plans? What about product launches - are they well coordinated and do they feel like they're executing at the level they want to be? How about sales enablement? 

Once you find out where the gaps are, I'd focus your next stage of research on those areas. Find articles, podcasts, webinars, etc. that cover those topics - PMA, Pragmatic Marketing, Product School are all great places to start!