Here are some great books to read to your preschoolers that introduce concepts of equality, difference, and friendship.
Hallinan, P.K. A Rainbow of Friends.
A story about the special qualities friends have, despite their differences or difficulties.
Howe, James. Horace and Morris but Mostly Dolores.
Three mice learn that the best clubs include everyone.
Raschka, Chris. Yo! Yes?
Two boys meet on the street and after exchanging a few simple words become friends.