Like others, I was intrigued by the title - let's face it, who wouldn't want to be able to learn anything fast and be good at anything in 20 hours? It certainly beats spending 10,000 hours of deliberate practice to become an expert.
However, whilst I really liked Josh's earlier book "The Personal MBA" and would fully recommend that you do purchase/read this as it is really very good, this was a disappointment. The most value in the book was, as others have said, pretty much common sense although I did find it useful to have this articulated in an easy-to-read list form. Unfortunately, this was the limited to the first couple of chapters and probably should have been confined to a blog post or website article. The vast majority of the book was devoted to a series of case studies demonstrating how Josh put the theory into practice and whilst this was useful in parts (I found the section on consolidating a newly-acquired skill via sleep and thus scheduling practice at the end of the day to be helpful), most of the book was uninformative and bordering on the dull.
I really wanted to like this book given that the concept of becoming proficient in anything in 20 hours has to be appealing, and because I thought The Personal MBA was first class, but it was just impossible given the content. Put in this way, if I'd read this prior to discovering The Personal MBA, I wouldn't have bothered to read another book by this author. And given the quality of The Personal MBA (regardless of whether you agree with his basic premise in the book that you don't need to spend £000's on a business school MBA), this is a real shame, and a warning to others of how easily you can damage a good brand through poor content.
I'd recommend that you check out The Personal MBA instead!The Personal MBA: A World-Class Business Education in a Single Volume