www.amazon.co.uk/Playing-Laughing-Learning-Children-Spectrum/dp/1843106086/ref=sr_1_5?s=books&ie=UTF8&qid=1302091965&sr=1-5 is a reasonable starting point.
Things that might help...(all generalising)
We see pretty well out of the sides of our eyes. Looking straight at someone to play with them is heckishly scary, thanks to the different brain wiring that sends eye contact signals straight to the "run! It's a monster and it's going to eat you" part of the brain, rather than the "ooo, who's this, is it a friendly person" bit.
We play alongside people rather than with them, but for us this is our preferred way of playing. We find it very strange indeed that others want to play 'with' us. See it as a difference rather than a fault, and it becomes easier to share our way of being rather than think we just don't know how to play.
If you want to share a game, it helps to know that, to us, play toys feel like our babies. Imagine someone coming up to you in the street and taking your baby out of the pram and playing with them. How bezerk would you go? Our brains put 'things' in the place where 'most precious person in the world' goes in your brain. So treat our stuff with respect and get our permission to play with it or move it etc. It'll help a lot.
Use our special interests. If we love cars, get our interest by getting us to draw cars, count cars, learn the colours of cars, play games involving moving the cars...then gradually teach us about different other vehicles...and about the shapes that make up cars (circles for wheels etc). The more you can base it on something we love, the more interest you'll get.
Take it slow, keep it as low-sensory as possible. If we're battling background noise and you chatting all the time, plus social demands, we'll shut down our ears and not listen. We can hear and see and sense three times more detail than you can, so for us, a little is a lot.
Enjoy!! Our world is absolutely fascinating. Wish you could experience it for yourself...