Which two books are those? The three campaign setting books he had stats in (Black Box, Red Box, and Domains of Dread)? The adventure module he was the central character in (When Black Roses Bloom)? The two novels and a short story he was in (Knight of the Black Rose, Spectre of the Black Rose, and "Rigor of the Game")? The gazetteer of Sithicus that talks covers Soth as the previous darklord of the domain (in Ravenloft Gazetteer Vol. IV)? All the other interconnected references to how Lord Soth influenced things in Ravenloft? Heck, even the crappy Iron & Blood Ravenloft video game had Soth in it.
I think you'll find a lot of designers who worked on Ravenloft that disagree with your statements.
Chris Nichols