I am SOOOO PLEASED to see the additional features and methods to read/write registers [in the Teensy-LC and 3.x. Maybe these have always been there in Arduino but I have not been able to find it.

Another thing I have been disappointed with is the Reference section in Arduino. Their website is rather non-detailed. I have not been able to locate a good - detailed - like most other language / syntax guides - language reference.

I'd pay for a good book if one is available.