I would be skeptical that this book was written by any 13 year-old, with or without autism. I'm putting it on my shelf next to "Three Cups of Tea" and "A Million Little Pieces."
This is the longer review I wrote on Amazon.com (under the pseudonym Bruno T.)
http://www.amazon.com/review/R1XCQAT74K4V41/ref=cm_cr_pr_perm?ie=UTF8&ASIN=0812994868&linkCode=&nodeID=&tag=
This is a good review...
http://www.spiked-online.com/newsite/article/13950#.UlIcQySxOVg
And here are some quotes...
"It is undoubtedly reassuring for parents of children with autism to discover in Higashida’s account a boy who not only sympathises with their difficulties, but also shares many of the familiar views of middle-aged, middle-class readers in Western society"
"It is when Higashida turns to the wider significance of autism that the moralising sentimentality of this book becomes fully apparent. Higashida observes that ‘I think that people with autism are born outside the regime of civilisation’. The message from the voice behind the alphabet board is that ‘as a result of all the killings in the world and selfish planet-wrecking that humanity has committed, a deep sense of crisis exists’. Higashida claims that people with autism are ‘like travellers from the distant, distant past’ who have come ‘to help the people of the world remember what truly matters for the Earth’."
"I believe that my son enjoys swimming pools because he likes water, not because, in the fanciful speculations of Higashida, he is yearning for a ‘distant, distant watery past’ and that he wants to return to a ‘primeval era’ in which ‘aquatic lifeforms came into being and evolved’."
"I fear that the translation and endorsement of this book reinforces more myths than it challenges. Like Mitchell, like other parents, I have spent much time pondering what is going on in the mind of my autistic son. But I have come around to agreeing with the pioneering Austrian paediatrician Hans Asperger that ‘the autist is only himself’ - there is nobody trapped inside, no time traveller offering redemption to humanity."