Master the Product Management Interview
The Ultimate List of Product Manager Interview Questions
Lenny Rachitsky
Lenny's newsletter is a valuable resource for all sorts of product management topics and now you can chat with everything he has produced! Awesome concept, and excellent execution.
Agile Product Manifesto
249 product portfolio examples
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Silicon Valley Product Group
Publishers of the books Inspired, Empowered, Loved, & Transformed. Marty Cagan is strongly associated with SVPG