Spectrum News in Wisconsin just interviewed Erin Falligant, the author of Joss's book series, about how she wrote Joss's books.
She said that for Joss's books, who she described as "very relatable... so lovable and ... resonating with girls," she became fluent in American Sign Language. She also had Joss's advisory board help her craft the prose in the book and correct any inaccuracies. In total, it took three years and 10 drafts to write Joss's two books- longer than an average Girl of the Year.
P.S: The last episode of Joss's stopmotion series on American Girl's YouTube just released! Go check all the episodes out if you haven't yet, because they're really awesome!