Ask HN: How do I get a better understanding of NoSQL databases

I have a reasonable understanding of relational databases. I am not so sure about NoSQL databases. Though I have used mongodb in the past. Can someone point me to resources to get a deeper understanding? How do I go about learning the trade-offs of choosing say a document store over a key value store. Also CAP theorem and related trade-offs. When is NoSQL better suited than relational etc.