Dominus: the original book called Torment is written by Ray and Valerie Vallese, many stated that this is a weak adaptation of the game's story.
the GOG .pdf uses the original text by Chris Avellone and Colin McComb and the novelization was created by Rhyss Hess. Since Avellone was the lead designer of the game I believe this is the best version to read.
also the PDF contains this paragraph:
"This novel has been composed from text copied from the computer game
Planescape: Torment by Black Isle Studios. It was turned into a novel by Rhyss
Hess, and typeset by Lucian Wischik. (There is a separate novel called just
“Torment”, based on the plot and characters of the computer game, and written by
Ray and Valerie Vallesse)."
. I didn't read either of them yet but based on the two books' endings this other version will be the one I will be reading...