People
We can and should learn much from other people. If I were to list all the exceptional people in our profession, I would never complete that list.
Martin Fowler
Martin Fowler has been and is a force in our profession. Besides authoring a ridiculous number of books, he was also one of the members who drafted the Agile Manifesto back in 2001. I recommend starting with his website, martinfowler.com, which has a ton of great articles on architecture, refactoring, agile, and more.
Recommended articles:
Robert Martin
Robert Martin (aka Uncle Bob) has been extremely prolific and influential. The SOLID principles were introduced by Martin in his 2000 paper, Design Principles and Design Patterns.
Check out his site: Uncle Bob Martin.
Jeff Atwood & Joel Spolsky
These two are the founders of Stack Overflow and Stack Exchange.
Jeff Atwood also maintains the well-known Coding Horror blog.
Joel Spolsky's site is Joel on Software. In particular, check out The Joel Test: 12 Steps to Better Code. It was written in 2000, but as much as things change, they also stay the same.