I recently bought my mom a copy of Being Autistic by Niamh Garvey @niamhgarvey .
She read it on her own time and took notes, and we made time to discuss it later. She asked questions, I answered. I tried to keep my answers concise, and not to rant. She still has a lot of gaps and misunderstandings but it's a start.
(It also led naturally into a conversation about our family history, including stuff around her divorce that I'd never known, which was a welcome surprise.)
It's written for children, so even grownups can understand it! Not kidding. The words are simple and accurate, avoiding the problems of infodumping and therapy language that plagues most other pop autism books.
https://www.amazon.com/Being-Autistic-What-Actually-Means/dp/1805011715