1. The central message of the book is that uncertainty is much more widespread than we realise, so even an investor who buys a high-quality stock can be speculating.
2. PE is almost irrelevant for making investment decisions. Dividend yield is even less important.
3. 12 rules that investors should follow (importance of holding at least 10 stocks, no more than 25% in each stock, avoid mechanical formulas, be quick to take losses / reluctant to take profits).
4. A weak economy does not mean a weak stock performance, although increasing bond exposure after a long period of economic growth and rising bond yields is advisable.
5. Management has much more power in making decisions despite shareholders formally owning a company (and voting on management). This is why a company with great management is much better than an average one (as a shareholder you have much less power to change management or impact their decisions).
6. One other interesting concept is that within a broad market trend (e.g. a bull market) there could be many other sub-segments some of which can be in a crisis (prosperity and depression may co-exist side by side).
The book has a good review of accounting for investors and key financial ratios, but anyone who has read 'Security Analysis' by B.Graham or more modern books on this subject, will not find anything new there.