I have not read the book, so I can't really say too much about the actual book itself. I will tell you, the autism world is full of controversy--from causes to "cures." Most of the "treatments" are very costly, and often not studied for effectiveness. Also, every child on the spectrum will progress differently & have different issues, and each child will benefit only from their own, unique plan of treatment.
Since your child is progressing & seems to be doing pretty well, I would continue what you are doing. Focus on sensory integration is my best advice, along with visual cues (PECS cards or sign language--along with verbal speech therapy). Also, what works for one child, may not work for another. Our daughter has always had delays and issues, so we don't feel her autism is because of vaccines or anything environmental (that said, we eat organic and use "green" cleaners).
If your child is not a picky-eater, then I would try the GFCF. ABA therapy has also been helpful for many kids (Floortime was what we did when our dd was very young). I do have some favorite autism books I can gladly recommend:
Ten Things Every Child With Autism Wishes You Knew by Ellen Notbohm
My Friend With Autism by Beverly Bishop
Making Peace With Autism by Susan Senator
The Autism Answer Book by William Stillman
The Autism Acceptance Book by Ellen Sabin
The Game of My Life by Jason J-Mac McElwain
The Out of Sync Child by Carol Stock Kranowitz and Lucy Jane Miller (and The Out of Sync Child Has Fun also)