While there are so many incredible books now, my favorites tend to be related to helping parents better understand child development. In no particular order:

The Conscious Parent - Shefali Tsabary

The Emotional Life of the Toddler - Alicia Lieberman

Raising Your Spirited Child - Mary Sheedy Kurcinka

The Whole-Brain Child - Daniel Siegel

The Explosive Child - Ross Greene

How to Talk So Little Kids Will Listen series - Joanna Faber, Julie King, Elaine Mazlish

Boys of Few Words - Adam Cox

The Highly Sensitive Child - Elaine Aron