She had just completed a Ph.D. in language, literacy and learning, and she was analyzing children's access to books in affluent and low-income communities.
One day, she saw something that stopped her in her tracks.
"I was at a school in a wealthy community, and they were getting rid of almost brand-new books because they didn't have room in the library," Constantino said.
She put the books in her car and drove them to a school in an underserved community.
Shocked by the disparity she saw in public school libraries, Constantino became determined to bridge the gap.
"School libraries are not funded well, and sometimes not funded at all," Constantino said. "If you're a child in an underserved community, you're left with boring, outdated, tattered, uninteresting books."
Word spread about Constantino's work, and books began piling up, literally.
"I could actually fit about 4,000 books in my car with paint to paint a library," she said.
Today, her nonprofit, Access Books, has provided more than 1.5 million books to public-school and community libraries across California.
The group has also refurbished close to 350 libraries in public schools, battered women's shelters and homeless shelters.
Constantino enlists volunteers who work alongside the community to create a warm, vibrant and welcoming library space. Students and their families, as well as school staff and teachers, come together to pitch in.
"We're working together. It's not a gift, it's a partnership. It really builds a sense of community," Constantino said.
In addition to book donations and refurbishing libraries, the group offers author visits with schools and holds an annual writers conference for kids on the UCLA campus.
"Our whole goal is to spread literacy and the benefits of literacy," Constantino said. "We want kids to feel part of this literacy club. And that all members are welcome. And the doors are unlocked. You just come on in."
CNN's Allie Torgan spoke with Constantino about her work. Below is an edited version of their conversation.
CNN: Every time you help a library, you provide about 6,000 books. Where do they come from?
Rebecca Constantino: Our books come from two sources. Donors purchase new, hardcover books, and we have groups that do book drives.
People have more books than they realize, and their kids have more books that they don't read than they realize. We have drop-off sites all over the city, and children collect for their bar and bat mitzvah. Local schools, Girl Scout troops, Boy Scout troops and churches all collect them. Ideally, people could replicate this anywhere in the country.
CNN: Who receives the books that your group collects?
Constantino: It's children who cannot afford to go and buy books. And it's children who don't have access to quality, public libraries. Some kids have never been to a bookstore. They don't have books at home; the source is the school library.
Our goal is really to provide high-interest fiction and non-fiction, because you learn to read by reading for fun. Kids want to read "Diary of a Wimpy Kid." They want to read books that are multicultural and oriented towards them. They want to see faces like theirs.
If one child discovers their imagination for us redoing that library, we were very successful.
CNN: What are some of the long-term effects of your work?
Constantino: I've spoken to children who have taught their parents how to read, who have taught their little brothers and sisters how to read, who have taken this experience of going to the school library and ventured out into a public library because their understanding is now you can check out books and borrow books and you can get a big stack of books.
If we provide high-interest books and a beautiful library space, they do come and they do read. We found that in the schools that we serve, circulation doubles and triples after we've come. So, we know that kids come to the library. And the books that we give, they're difficult to keep on the shelf because we're giving books that kids are really interested in.
My goal is that kids can always have a friend in a book and they can always turn to a book to find comfort, to learn something.
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