The Transgender Child: A Handbook for Families and Professionals, by Stephanie Brill — recommended by blogger loveonastick.
Becoming Nicole: The Transformation of an American Family, by Amy Ellis Nut — describes to story of identical twin boys. Nicole insists she is a girl from a young age. The books describes the effects of Nicole’s gradual transition on her family, the changes and struggles they undergo, problems in gaining acceptance of Nicole at school, and ending with Nicole in her (late?) teens contemplating SRS (sex reassignment surgery). My husband’s men’s book club read it. Two of the guys later came up to me and said reading the book gave them understanding of what it means to be trans for the first time. The story is moving, although it is occasionally broken by chapters explaining the scientific understanding of transgenderism.
Explore: Transgender Children and Youth, a resource of the US-based Human Rights Campaign (HRC). This page has links to multiple resources for parents of trans and gender-expansive kids.
Transgender Resources, several types of resources provided by PFLAG, an organization that promotes the welcoming and LGBTQ folks. It also has many chapters across the US that offer group support to parents and friends.