Common playground problems and how to help:https://www.understood.org/en/friends-feelings/child-social-situations/playgrounds-playdates/8-common-playground-problems-and-how-to-help#slide-1
Books:
How to Be a Friend by Laurie Krasny Brown and Marc Brown
Making Friends by Fred Rogers
My Friends by Taro Gomi
Amadeus Best Friends by Eli Cantillion
How Humans Make Friends by Loreen Leedy
Do Unto Otters: A Book About Manners by Laurie Keller
Personal Space Camp by Julia Cook
Simon's Hook: A Story about Teases & Put Downs by Karen Gedig Burnett A good resource for 3rd and 4th graders who need ideas for how to deal with teasing and put downs.
Sometimes I Feel Like I Don't Have Any Friends (but not so much anymore) by Tracy Zimmerman & Lawrence Shapiro
You Can Make Friends by Lawrence Shapiro & Greg Elkins