Your child with autism is not the child that you thought you would have.
The question posed is whether or not you should mourn for the child you did not have. The author of the book has a refreshing view; he tries to celebrate the child that he does have, instead.
After many years of effort, I have been able to approach that attitude with some success.
When you give birth to a baby that is, from the start, not quite right in some way, you can grieve, accept and move on to face the challenges ahead. However, if your baby seems quite lovely and intelligent and you lose her to autism at the age of 1 or 2, then it seems as if the child she was has died. You have no time to mourn her death because you are thrust into dealing with enormous challenges. You push the feelings aside and try to keep your head above water searching for answers and trying therapies, dealing with the day-to-day.
Once in a while you can glimpse that former child in the child you have; there is always the hope that you can get her back. So you put off the mourning and accepting a bit longer.
When I have allowed myself to mourn, (or wallow in the grief more like), it can be paralyzing. My thoughts become circular and pointless and I am unable to productively deal with the crisis at hand. And there is always a crisis.
That is the challenge: to break the attachment to the hope that you will get the first child back and relish the rewards of who she has become. Or not become. Not riding around in cars with boys, not taking illicit drugs, not doing the scary things we associate with teenage girls.
And when you do follow her around you find someone who is quite lovely indeed.