KMart traded way below the book and people bought it, then the management took it all into bankruptcy and the stock went to zero and ceased to exist, when they came out of bankruptcy they issued new stock in the $27 range and everything was fine once again... unless you had the old stock...
Book value might not be known, although it may appear to be knowable. Consider a drilling machine. It might be on a company's books for $ 10,000 but if I had to sell during a recession I might only get $ 100 for it.