I have both S.T.A.L.K.E.R - Southern Comfort & the sequel S.T.A.L.K.E.R - Northern Passage in physical form on my bookshelf. Pataki originally published under the name of John Mason. Been several years since I read them.
Vaguely recall there were some copyright issues, where GSC initially gave the author permission, then removed it, so he re-released the books removing the stalker references.
Every so often I do a search to see if any of the officially licenced novels have been been translated (either fan or officially). Not yet found any, so these (and a couple of graphic novels) are the only written forms in english we have that I know of.