When I started my adventure to buy a bike, one enthusiast's first piece of advice was to get this book. Then, before doing business with a shop, he recommended I "ask one of the mechanics (not the person working the register) if they sell Sloane. Don't ask for it by title, just ask if they sell Sloane. If they don't know immediately what you're talking about, there's a good chance they don't do good work. Tell them you forgot to put change in the meter and walk out and don't come back." I found this to be good advice.
The other reviewer has pretty much said it, but I got more from my time spent reading this book than talking with any shop personnel (no question) or almost any bike owners. Considering the cost of investing in a bicycle, and the cost of this book, there's no excuse for any bicycle owner to not own this book.