The S&P 500's price-to-book ratio has reached an unprecedented level of 5.3x, marking a new record for the index. This metric, which compares a company's market value to its book value, is often used by investors to assess whether a stock is over or undervalued. The current high ratio suggests that the market may be overvalued, raising questions about the sustainability of current stock prices and whether this time will differ from past market peaks.