Hey all:
Here are a couple of sources I've been using to learn about Arduino (I know, dumb...) and the ESP8266:
https://leanpub.com/kolban-ESP32https://www.udemy.com/The first is a *free online book by somebody who really knows about computers, and who has written 400+ pages about setting up and using the ESP8266.
The second has a number of courses related to electronics in general and microcontrollers in particular; some topics to browse might include Arduino, IoT, ARM micros, robotics and home automation.
*Neither source is strictly free, but Mr. Kolban's download link wouldn't let me pay him any money, and Udemy is constantly running introductory discounts for new students that drastically slash their prices, so it might pay to check it out.
Apologies if these have already been mentioned; this thread is getting kinda long.... Which shows that there is a lot of interest.