I've read the first three chapters, which are free. I think the book is pretty rigorous in terms of back-testing his recommended income harvesting strategy using different data sets. I do see some potential issues for early retirees, and wonder if others have similar concerns.
First, the recommended prime harvesting strategy has you periodically sell 20% of your stocks and buy bonds. You do this when your stock portfolio grows to over 20% of its inflation-adjusted starting point. This will result in an occasional large sale of stocks, and it seems like it would be tax-inefficient. You might do this within your 401k, IRA, or other tax-sheltered account. But the point is to use bonds to fund annual income withdrawals, which you can't do from your 401k if you're an early retiree. The author states that he doesn't believe that tax considerations will drive choice of income harvesting method, but does not back that up. It seems to be an assumption that he makes, which may be a good one for all I know. I'm not a CPA. Maybe you can smooth out such large income variations under the tax code somehow.
Second, the author specifically states that he is looking for a harvesting strategy that maintains income, but not necessarily portfolio growth over a 30-year retirement period. He recommends the prime income harvesting method based on this assumption. But this may be a poor assumption if you're an early retiree. If all goes well, after 30 years you will have another 30-year retirement to fund. Some of the surveyed income harvesting methods showed a lower SWR, but higher average ending balance after 30 years. An early retiree might be better off with one of those methods.
Still considering buying the book, these are just my initial thoughts.