TMZ has learned Heidi Montag is downright furious with Spencer Pratt's new claim that he plans to write a tell-all book about her private life -- and is now threatening legal action to stop him.

We just spoke to Montag who tells us, "This is exactly why I left him ... Right now I'm looking into my legal options to stop it."

As we previously reported, Montag filed for divorce back in July -- and plans on making their split official on Valentine's Day.