EV3 resources

LEGO documentation, including firmware, hardware and communication kits. Also the firmware source code can be downloaded from this link.
https://education.lego.com/en-us/support/mindstorms-ev3/developer-kits
- Firmware developer's kit has all the opcodes for sensors, motors, etc and is the most useful for API development and extension

A detailed description of the specs for various sensors.
http://ev3.fantastic.computer/doxygen/types.html

Blog by Christoph Gaukel
http://ev3directcommands.blogspot.com/2016/01/no-title-specified-page-table-border_94.html
- Invaluable source for understanding how to put together direct commands

LEGO's building instructions:
https://education.lego.com/en-us/support/mindstorms-ev3/building-instructions
- Good place to start for building and designing a robot

API's written in other languages:
python:
https://github.com/ChristophGaukel/ev3-python3
small basic:
https://github.com/c0pperdragon/EV3Basic