Hello - I don't know of any single parent storylines, but we have a couple mom/child books in our collection. I Love You So Much by Marianne Richmond is one of my favorites. There are some pages of father/son or father/daughter, but mothers are on most of the pages. Also, there is a mother/daughter pair that start and close the story.
The other title is If You Hold My Hand by Jillian Harker. It's about a boy bunny who is more confident and willing to try new things with his mommy bunny holding his hand.
Have you ever read A Mother for Choco by Keiko Kasza? It's not my favorite, but many people seem to like it. A small bird is looking for his mother, and he asks all kinds of animals if they are his mother. He finds a bear who becomes his mother. She happens to have other children, too, who don't look like her. It's supposed to be a story good for adoption, but I find the rejection that Choco experiences to be offensive. I mentioned this to another parent in the adoption community, and she thought that I was crazy. Maybe I am ;)
Anyway, another title that talks about family differences is We Are All Alike, We Are All Different by the Cheltenham Elementary School Kindergartners. This is a favorite in our home. It talks about how we are all people but we look different. We all eat, but we like different things. You get the idea. I find it to be simple and positive, perfect for toddlers.
Lastly, I just found a new book this week: I Like Myself! by Karen Beaumont. My son talked about it all week. I imagine that he will enjoy this one for a few years. "Even when I look a mess, I still don't like me any less, 'cause nothing in the world, you know, can change what's deep inside, and so..." I really like this book. The main character is a girl, too. It has no pictures or mention of parents, fyi.
Happy reading!