This (3.4 MB) is the reference manual I have been using, and it's probably best if you use it too.
The official chip datasheets are published by Microchip Technology Inc. who bought Atmel in 2016.
Microchip's main page for this chip has the fully up-to-date information from the manufacturer. None of the documents looks very similar to the one I've been using when making these videos. Even the section numbers are different.
Just in case you're interested to see what the native assembly language of this processor looks like, here (2.5 MB) is its reference manual.
Similarly, since I based the ArdEx assembly language on the MSP430, you might be interested to look at the MSP430x2xx Family User's Guide.