We count a book as "about" if the main character/subject is a person of color, or if we are able to determine based on examining a book that a person of color features significantly in the narrative. So a novel in which the main character is white will be included if we are able to determine a secondary character of color is important in the story. We do not count a book if the principle character is white and there are a range of secondary characters, including characters of color, but none of the characters of color seem to play a significant role. This is, of course, somewhat subjective; we talk about the books that we can't easily discern. We do not want to misrepresent a book as having multicultural content; likewise, we make every effort not to miss those that do.

Other links of note:

At the CCBC: Multicultural Literature 2014.
At UW-Madison News: UW-Madison center sees hope for more diversity in children’s books. "“We’ve seen the same old story year after year,” says CCBC Director Kathleen T. Horning. “If the numbers rise one year, they’re likely to fall back the next year. It’s as if there has been a glass ceiling when it comes to diversity in children’s books. The numbers have stagnated throughout most of my career here.”"
At CCBlogC: I See White People. "So to get back to Roger's semi-facetious response, here is the big picture. Of the 1509 books published in 2013 that we have received so far, 1183 (or 78.3%) are about human beings. If we subtract the 326 books about nonhuman characters from the overall total and just figure the percentages of books about people of color among the books with human characters only, we still get a fairly dismal number: of the 1183 books published so far in 2013 about human beings, 124 of those books feature people of color." This is from 2013, but if you haven't read it, do.
At PW: CCBC Stats Show Children’s Books Shifting Toward Diversity. "And at the end of the day, Horning says the key to having more diverse books for children is in the hands of consumers. “The books have to sell,” she says. “Publishing is a business. If diverse books sell well, there will be more published. We have to help make that happen by buying the diverse books that are out there. Our kids can’t wait.”"
At NCTE: Resolution on the Need for Diverse Children's and Young Adult Books. "This proposed resolution builds on the 2006 resolution and the two referenced position statements, although greater emphasis is placed on the publication and production of literature to intentionally reflect human diversity. Now is the time for NCTE to advocate for increased publication of culturally diverse literature that reflects human, cultural, linguistic, and family diversity."
At Brooklyn Arden: 2014 Statistics on Children's/YA Books by Race/Ethnicity. "I am delighted to see the year-over-year almost-doubling of the number of Black and Asian book creators. But still: We can do much better, people."

Report: Children's Books by and about People of Color Published in the United States in 2014.