Friends Learn about Tobin and Tobin Learns to Make Friends by Diane Murrell
These books use a train theme that is appealing to lots of little guys on the spectrum.
Friends Learn... is a help for introducing classmates to some of the traits of ASD.
Tobin Learns... goes from the perspective of the child with ASD learning about making friends. Both books are pretty simple and appropriate for using with little kids.
If your little one tends to be somewhat aloof and doesn't do much in the area of reciprocal play, you might want to look at Giggle Time by Susan Aud Sonders. This is a good book for establishing play routines. You will find lots of fun ideas for activities. It has a major focus on social communication skills.
For children with sensory issues that might be impacting social skills, you might want to look at The Out-of-Sync Child by Carol Kranowitz. Sometimes little ones have problems with the proximity of others, with the sensory overload of the environmnent, or with sensory issues involved with play materials. This book can help you pinpoint some issues and gives you ideas for helping a child learn to cope so he/she can enjoy childhood experiences more fully.
Learning to play appropriately with toys is so important to early development of social play.
A good book with lots of great background information about teaching playskills combined with ideas for teaching about specific types of toys would be the book Teaching Playskills by Melinda J. Smith.
Another book along the same lines is Playing, Laughing and Learning with Children on the Autism Spectrum by Julia Moor. This book looks at learning to play table top games, physical activity games, outdoor games, water play and more. Again, kids have a hard time socially connecting unless they can play games and use toys like other kids do. Even as adults, we connect with others with similar interests. With little kids, those interests naturally revolve around toys.
Imitation and initiation skills are usually difficult skills to pick up for children on the Autism Spectrum. The book Pivotal Response Treatments for Autism by Robert and Lynn Koegel has some great , somewhat scholarly, information on these topics.
Some early learners need visual supports to learn play routines. Great Prairie Area Education Agency has a link to a resource called Play Routines. Here you will find a free download of hundreds of visual play routines and planning guides. http://www.gpaea.k12.ia.us/media/31825/play%20routines.pdf
I hope you will find some ideas to get the little ones off to a good start. Play away!