With the market at all-time highs, many investors are left wondering if stocks are still attractive investments. Based on PE ratios, the market's current level of 17.5 if slightly above the historical average, but not excessively so. Thus, if corporate profits remain strong, valuations could remain stable (so says some analysts). See article and video here, Fidelity. The video also discusses the difficulty of market timing, which is a good reminder for most investors.
Dolvin, Steven D., "All-Time Market Highs and Market Timing" (2014). All Chapters. 110.