As other reviewers have pointed out, this (sixth) edition of Security Analysis includes a reprint of the popular 1940 edition (Warren Buffett's favorite), minus some chapters. The deleted chapters, which can be found in the CD that accompanies the book, make room for (new) general introductory remarks to this edition and also to the eight major parts of the book, written by a variety of modern commentators. If all you really want is the 1940 edition, then the better choice for you is probably to buy the currently in-print reprint that's available (for a lower cost that this sixth edition). You could also buy one of the books actually printed back in 1940, but that will set you back big bucks, because original versions of Security Analysis are collectors' items today. For example, a good copy of the first printing of the 1934 edition easily runs into five figures.
Concerning the 1940 edition (or just the 1940 chapters contained in this sixth version), other than for the references to corporate examples from the 1920s or 1930s, the content is amazingly relevant to today's investing. I had read Security Analysis (the fourth edition) many years ago, and I had forgotten just how clear, precise, insightful and truly sophisticated Graham and Dodd were. Remarkably, in many instances this 1940 edition does a better job describing 21st century investing issues than the majority of material written today.
What you get with this sixth edition that's not available in the other editions are a short (two page) foreword by Warren Buffett and 10 essays by some of the most well-regarded modern investors and authors. Indeed, it was an honor to be asked to contribute to the sixth edition. The essays run about 15 pages, on average, and many of them are highly informative and useful. Those written by Seth Klarman, James Grant, Roger Lowenstein and Bruce Berkowitz were my favorites. As good as they are, though, they basically provide useful insights and more modern applications, rather than plow much new intellectual ground. It's hard to improve much on Graham and Dodd, even after 70 years. If that seems hard to believe, read the book and see for yourself. Finally, if you haven't already read the 1940 edition, which book should you buy--a 1940 reprint or this sixth edition? My choice is the sixth edition.