For web applications, understanding the concept of state and managing it takes you a long way towards building a scalable system (on a related note, any time spent really understanding HTTP is time well spent). It's very useful to read books, presentations and war stories, but for me, reading code in applications designed for scale was far more important. I strongly recommend the Amazon Dynamo paper. Read and think about the design decisions they made. Then go read the Cassandra code for the actual implementation (pretty much, in terms of the big ideas, at least as it was back in 2008).

As with any learning it's very hard to truly grok the importance of loose coupling, service oriented architecture, embedding statelessness in the architecture etc., unless you try to build something, get it wrong and then fix it. So, the very best way to learn would be to put yourself in a place where you get an opportunity to do that.