The hardest part about gardening for me is that it feels unintuitive. Some seeds need to be planted at 2 inches, some need to be planted at the surface. Some like lots of organic material, some don't. Some like it wet, some like it dry; some like sun, some like shade -- There's no underlying set of rules, so what you've got to do if you want to garden well is memorize every plant's preferences. This book is essentially a plant-by-plant guide, and it's pretty good at describing how to plant, harvest, and sustain a garden with the plants it covers.
However, there's far too much emphasis on the "free" element that's in the title. For example, the author talks about how he traded a half-hour of his labor in exchange for three bulbs of garlic. Even at my expensive California Whole Foods market, that's about $1.50 in garlic in exchange for a half-hour of time, valuing the author's time at $3.00/hour. That's a very bad rate, and time is certainly not free. The book has a fair amount of bad math, and would be improved if the "free" part were de-emphasized.