Faye Resnick -- Kyle Richards' best friend and who regularly appears on "Real Housewives of Beverly Hills," claims a strange person has been following her and even left her a threatening letter.
Law enforcement tells TMZ, Resnick -- who also wrote a book accusing OJ Simpson of murdering her then-best friend Nicole Brown SImpson -- called police to report someone was tailing her.
Resnick -- who has also modeled for Playboy -- told cops someone even snuck on to her property and plastered a threatening note on her car.
We're told police are on the hunt but don't have a suspect.
In the meantime, cops have been doing extra patrols on Faye's block.