One of the best ways Product Marketers can provide value is through market and consumer insights. Often times Product Management teams are laser-focused on understanding existing customers and developing features to meet their specific needs. Product Marketers can provide perspective on the broader market and the needs/use cases for segments that a company hasn’t yet targeted. Product Marketers are testing different messages and value-props in-market every day and those learnings can help guide a Product Teams' future roadmap.
I would also check out Product Marketing Alliance. They have good blog content, a podcast, and a really active Slack Community. One thing I like about their Slack group is that it includes tons of Product Marketers from outside of the US/Canada, which adds great perspective. Another resource I find helpful but is more specific to the Mobile Apps space is the Mobile Presence Podcast.
100% agree with Suyog. Nothing we do exists in a vacuum and all of the positioning and messaging we bring to market should be looked at from a brand lens to ensure consistency. Ultimately, the consumer is not going to differentiate what’s brand marketing and what’s product marketing. It’s all the same to them!