If you search the Children's Books listings on Amazon.com for "autism" you turn up 759 results. Not all of these are actually books FOR children - many are aimed at adults teaching ASD kids or teaching neurotypical kids about autism. But there are quite a few picture books out there.
Many of these are targeted for typical siblings of kids with autism: "All About my Brother," "Autism and Me: Sibling Stories," "My Sister has Autism," and so on.
Some are aimed at typical kids who may encounter an autistic child at school. They have titles like "The Autism Acceptance Book: Being a Friend to Someone with Autism" or "A is for Autism, F is for Friend" - titles that sound like the Brussels sprouts of literature: I'm sure they're good for you, but I don't know if I could get past my gag reflex.
Then there are books that seek to explain autism to adults, which for some reason come up in a search for children's books: "Ten Things Every Child with Autism Wishes You Knew" and the like
There are some exciting-looking stories told from the perspective of an ASD kid, appropriate for older children and teens, like Baj And the Word Launcher: Space Age Asperger Adventures in Communication or "In His Shoes: A Short Journey Through Autism" (looks very promising). I don't know what population they are marketed for.
There are very few books for autistic children. Searching Children's Books for "autism, ages 4-8" produces 67 results on Amazon, but a very small fraction of those would be appropriate for a kid like Ryan. Here are the more promising-looking ones, mostly for ages 4-8:
I Am Utterly Unique: Celebrating the Strengths of Children with Asperger Syndrome and High-Functioning Autism by Elaine Marie Larson and Vivian Strand
Different Like Me: My Book of Autism Heroes by Jennifer Elder and Marc Thomas
Why Does My Belly Hurt? A book for Children with Autism and No More Macaroni and Cheese. A book for children with Autism. by Janelle M. Love, MD, and Kim Taylor - these explain the GFCF* diet and why it is important to stick to it.
When My Autism Gets Too Big! A Relaxation Book for Children with Autism Spectrum Disorders by Kari Dunn Buron
Disabled Fables: Aesop's Fables, Retold And Illustrated By Artists With Developmental Disabilities
Oliver Onion: The Onion Who Learns to Accept and Be Himself by Diane Murrell
While there may be 700+ titles on Amazon, the only one of them I've seen at my local book store is "My Book of Autism Heroes." It's cute, but Ryans a good 5 years from being ready for it.
So I think I've found a niche that needs filling. Any illustrators out there want to chat with me?
* gluten-free, casein-free (no wheat, no milk)