I like pretty much everything David Sparks makes. He has good insight and generally soothing style. But, like many folks I've mostly given up on Apple's iBooks platform. I'm happy to see David transition to an all new learning platform for his wonderful field guides.
Siri Shortcuts is a terrific new automation tool in iOS 12 but it comes with a fairly steep learning curve. The least of that curve is just understanding what value there is in even using Siri Shortcuts. David's new automation course is a great primer and a good way to get some wood behind the Shortcuts arrow.
I already own the OmniFocus Field Guide but David keeps making it better so I'm going to keep recommending it. There's some substantial changes in the new OmniFocus for Mac and iOS that will probably make most users change their methods. I was already a big user of tags before Omni got on the bandwagon but that doesn't mean there aren't clever new ideas for me to consider in David's course.
The new version of the Field Guide adds lots of new content for managing tags. There are also several other updates for the new OmniFocus features and an entire video about Siri Shortcuts for OmniFocus.
You can watch the videos in the browser or download them for offline viewing. It's a good platform for a training product that I know has a long future.