Rather than spending time reading about autism you can probably better help this child by playing with him and doing stuff for his parents so they have more time to play with him
In my (very limited) experience autistic children aren't very interested in playing with people, so a minimum of reading might be useful if only to understand what kinds of "playing" are likely to interest an autistic child, and also which kind of "playing" have chances of slightly improving his communication/interaction skills and interests.
There's an autistic three-year-old that often plays in the sandbox with my kid - he's much more interested in playing with a single toy than in the other kids or his parents; unlike my 20-month-old who evaluates toys by how much attention the other kids are paying to them (resulting in the classical "everybody fight for the shovel, five minutes later everyone fights for the truck"), or uses toys to bribe other kids (preferably bribing with someone else's toy). The autistic kid can eventually tolerate playing with another kid (like, both fill up the same bucket), but has to be prodded to do so, and will usually end up going back to playing alone. The only "game" I've seen him play with someone else is being chased around by his parents, which he seems to find fun.
so a minimum of reading might be useful if only to understand what kinds of "playing" are likely to interest an autistic child,
One theory is that you start with what the kid will do without prompting and then gradually introduce yourself into his play using trial and error to identify actions he appreciates and responds to. This is called floortime. The other main paradigm is Applied Behavior Analysis.
First, a short personal note to make you understand why this is important to me. To make a long story short, the son of a friend has some atypical form of autism and language troubles. And that kid matters a lot to me, so I want to become stronger in helping him, to be able to better interact with him and help him overcome his troubles.
But I don't know much about psychology. I'm a computer scientist, with a general background of maths and physics. I'm kind of a nerd, social skills aren't my strength. I did read some of the basic books advised on Less Wrong, like Cialdini, Wright or Wiseman, but those just give me a very small background on which to build.
And psychology in general, autism/language troubles in particular, are fields in which there is a lot of pseudo-science. I'm very sceptical of Freud and psychoanalysis, for example, which I consider (but maybe I am wrong?) to be more like alchemy than like chemistry. There are a lot of mysticism and sect-like gurus related to autism, too.
So I'm bit unsure on how from my position of having a general scientific and rationality background I can dive into a completely unrelated field. Research papers are probably above my current level in psychology, so I think books (textbooks or popular science) are the way to go. But how to find which books on the hundreds that were written on the topic I should buy and read? Books that are evidence-based science, not pseudo-science, I mean. What is a general method to select which books to start in a field you don't really know? I would welcome any advise from the community.
Disclaimer: this is a personal "call for help", but since I think the answers/advices may matter outside my own personal case, I hope you don't mind.