Amazing that the title is true, it is the only investment guidebook you'll ever need!
I've often looked at the bookstore for other investment guidebooks, and read a fair number of them, but this book is by far the best. This book is at least two books in one. First, Tobias tells you some common sense things to do before sinking thousands in stocks or other investments. I loved this section of the book, and have gone back to it to again and again for clarity in monthly money management. The rest of the book goes through the menu of investment options and shines a bright light on each one. I learned from this section of the book, and ultimately concluded that Vanguard index funds were the best option for me. Once someone clearly explains to you that it is irrational to try to pick individual stocks that beat the market, you can't help but agree. Sure it is a lot more fun to pick stocks (and Tobias explains this side of it too), but it just can't really be a rational choice for the average individual investor that does the honest yearly math.
The only minor problem I have is that Tobias seems to dismiss bond funds. I think Vanguard Wellington (VWELX) and Wellesley (VWINX) are both really "index" funds that include bonds (VWINX about 2/3 bonds and VWELX about 1/3 bonds), though they don't fit the definition of an index fund. Both should be mentioned in this book because they are solid performers, especially during volatile stock swings. At the time of writing this, VWINX is up 7.39% YTD compared to the Total Stock Index (VTSMX) down YTD. VWINX also has a mere 0.28% expense ratio (0.21 if you've got 50K for Admiral shares) -- for a single fund with stock exposure, few performed better this past year than VWINX (full disclosure: today I only own VTIAX and VFIAX).
Overall this is a great book that delivers and can actually be used as a reference throughout life. I can't think of any other book I own that I've thought more often, "Where is that book?"