For the first time in decades, there’s a challenge to the hegemony of the biggest trade publishers in the US. As Sourcebooks continues to grow, it is now the fifth largest publisher in the country by print units sold, breaking the longstanding Big Five, and pushing Macmillan into sixth place, according to publishers’ internal analysis of data from Circana Bookscan (which does not itself publish market data that excludes distribution clients). While Macmillan is still considerably larger overall, with far higher ebook unit sales and an established, successful audiobook division, this is the first time an independently-run house has challenged …