Searching with Google returned only 3rd party documents (i.e. from chip vendors or university lectures). Is there a main document of such specification, similar to USB?
The original standard has been written by the Telecommunication Industry Association (TIA). Its name is EIA/TIA-232.
But you must pay to get it. You can buy it on TIA's store. This is the reason why you don't find it on the internet: publishing it is illegal.
Anyway, all the necessary information required to implement an RS-232 device is easily available on the internet and on various 3rd party publications, as you noticed. I guess TIA doesn't make much money from this standard anymore.
(and at this price, I wouldn't buy it)
Here are some useful resources: