His blog has a ton of new content on DMN, starting with a great piece that compares the path of the DMN standard with that of BPMN, which is considerably more mature. He discusses the five key elements of DMN, then goes into each of those in detail in the next five posts: Decision Requirements Diagrams, Decision Tables, FEEL (a new expression language developed for DMN), Boxed Expressions and the Metamodel and Schema. It’s really interesting to read his analysis comparing the evolution of the two standards: there was a time when everyone thought that BPMN was just about the visual notation, but to make it really useful, the interchange format and execution semantics have to come along at some point. Still, it’s useful to get started in DMN now with DRDs and decision tables, since that at least makes the decision models explicit instead of being buried in text requirements.
Once you’ve brushed up on his posts covering the five key elements, you can also read about conformance levels that vendor can choose to implement, and what didn’t make it into DMN 1.1, which is the first real version of the standard.
He doesn’t pull any punches in his discussion, and is not very complimentary on some aspects of the standard and how some vendor choose to implement it. Just as he is with BPMN. 