1. Scrum the art of doing twice the work in half the time- it's important to understand not only what to build but how to build it quickly.
2. Shape Up - https://basecamp.com/shapeup - it's important to understand tradeoffs and what is realistic
3. Blue Ocean Strategy - It's important to differentiate and see opportunity
A lot of PMs like to write and work out their ideas online (for better or worse). There are a ton of free and semi-free resources if you search around Substack and Medium. A lot of the VCs (e.g. Sequoia, Andreessen Horowitz) have great content as well. Reforge is also a great resource for information. Hope that help!