In the recent census data set, as of 2012 only 20% of households with children include married couples, down from 40% in 1970. This statistic does not discount the involvement of either parent but is one of many data points that show the world around us no longer fits into a two parent, heterosexual, one race family. The books listed here highlight or briefly mention different family dynamics, specifically single parents.
Half Magic by Edward Eager
A great book for beginning chapter readers, it is about a magical coin that only grants half your wish...so if you want the entire wish you have to wish twice as hard. It follows four children whose father recently died and thus their mother is busy working sun up to sun down, which leaves them to their own devices, mostly trips to the library :) I love this book because it does not focus entirely on their single parent family but sibling relationships.
Here are some more books that are for pre readers to high schoolers:
Pre/early reader: Love is a Family by Roma Downey
High School: Everything I Never Told You by Celeste Ng, this hits on single parent home, mixed raced families and homosexuality.