This is probably the only book on electronics that you need. Really you could probably learn all of this on the web, but this puts a summary of everything all in one place. In some sense it's like an electronics textbook, but it's better than that because it's much more howto-oriented. And for $20 or so it's an amazing deal.
My main complaint is that the chapter on theory is a bit weak in making the math comprehensible. The main problem there is that in some places a new formula is introduced that uses a variable such as Q but Q is never defined. If I were writing this then I would create a callout box every time a new formula was mentioned that clearly spelled out what each variable is even if it seems redundant. For example: "Ohm's Law: V=IR. V=Voltage (Electromotive Force) in Volts, I=Current In Amps, R=Resistance in Ohms. I depends on V and R." And do that for every single one. This would make it much more useful for reference when you need to look up a formula and can't remember what Q is supposed to mean.
But the main point of this book is the non-theoretical information that it provides. For example, the real world models of various components like capacitors and wires and how they look in schematic form when stray inductance and leakage current resistance are added. Also, the chapter which is almost like a rewrite of the theory chapter but with most of the theory replaced with the details of various types of real world components that you usually don't think about (such as different resistor and capacitor technologies) is extremely useful. Most of these little details tend to get left out of things like textbooks, perhaps based on the thinking that they're too irrelevant.
Though I wish some of the math stuff had been handled better, anyone trying to build electronics in the real world should buy this book. After buying this I really don't feel like I need any other book.
Lastly, some reviewers mentioned the thin paper. I didn't find that to be a problem. I used to buy some "low cost developing world editions" of books that were printed on paper not much better than newsprint and this book has much better material quality for not much more money than a comparable low cost developing world edition of a textbook.