One thing to note is the claim of teaching oneself the hiragana in 3 hours. I followed the time instructions, and I truly did learn the hiragana in about 3 hours, spread out over a couple days. However, I also practiced off the clock, and since completing the book I have practiced a lot to keep the information imprinted in my memory. I think that this is key: the book will teach you the hiragana in 3 hours, but to truly know it you will need to practice practice practice.
eBook Remembering the Hiragana