Most if not all of the standards shown there are freely available here, if you want to read more:

https://www.itu.int/rec/T-REC-V/en

Here's V.8bis, for example:

https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-V.8b...

As far as standards go, the ITU ones tend to be pretty readable.