Intercom on Product Management - The ultimate resource for aspiring product managers
Product School - The global leader in product management training
Getting Real - The smarter way to build web apps. A free book by 37signals.
know/how Product Managers - A weekly newsletter with 5 practical tweet-length PM tips
The Manager's Handbook - Comprehensive guide to becoming a great manager
PM Starter Pack - Practical guide on how to get started in product management