John Travolta : Sued by Author of "Bathhouse" Sex Book

John Travolta is being sued by an author who claims Travolta spread lies about his mental health in order to diminish his credibility ... while the author was trying to sell a book in which he claimed he personally witnessed Travolta engaged in gay sex at various bathhouses.


The man behind the lawsuit is Robert Randolph -- who wrote a book entitled, "You'll Never Spa in This Town Again" ... detailing Travolta's alleged sexual exploits with other men, including Randolph.


In his suit, Randolph claims ... as he was releasing his book, Travolta and his high-powered attorney Marty Singer (pictured above) unleashed a P.R. attack ... in which they falsely claimed Randolph had mental issues and spent time in a mental institution.


Randolph claims the attack was concocted to persuade the public that he's an unreliable source ... and they should NOT buy his book. Randolph claims Travolta's plan worked ... and retailers were deterred from selling his book.


Randolph is suing both Travolta and Singer for unspecified damages.


So far, no comment from Travolta. Ditto for Singer.