For the kids, adopted or not:
God Found Us You (domestic or international adoption, good for transracial)
Shaoey and Dot (international adoption, particularly from China)
In My Heart (domestic or international, great for transracial, not specifically about adoption per se, but about an adoptive family and REALLY good for attachment)
Tell Me Again About the Night I Was Born (for domestic newborn adoption only, not transracial)
We have the top four picture books and recommend them all.
If you are adopting transracially, it's also important to have a variety of books that show kids of different ethnicities. Kids need to see people who look like them, in books and in real life.
Also, books with attachment/love themes are important for promoting attachment (such as In My Heart, listed above). There are many great picture books that are not adoption-related but have great attachment themes such as Karen Katz books, I Love You Through and Through, the Llama Llama series, etc.) which paint a picture of what healthy parent-child relationships look like.
And for the parents, my top picks:
Attaching in Adoption (really for any adoption other than newborn adoption, if you can only read one adoption book, this should be it)
I'm Chocolate, You're Vanilla: Raising Healthy Black and Biracial Children in a Race-Conscious World (really informative read, particularly for a Caucasian parent of an African-American child)
There is No Me Without You (by an adoptive mama about Africa AIDS epidemic, really helps understand the situation there, emotional read but worth it)
Toddler Adoption: The Weaver's Craft (specifically for toddler adoption, author mostly focuses on international, but some could apply to domestic)
With Eyes Wide Open (not a book, but a great online learning course, Billy & I went through this together and learned so much, specifically for international adoption, really helpful in understanding how hard international adoption is from the kids' perspective and how to anticipate their needs)