College textbook publishing is transforming from a model of static textbooks to a modern model of customizable textbooks. Customization may involve reconfiguring content, combining textbooks, authoring one's own content, adding notes to content, and more. As such, publishing is moving away from a model of selling static textbooks, and toward a model of providing a library of content from which instructors can build a course. This Full Paper provides data for one digital-only publisher, zyBooks, on the prevalence and trends around reconfiguring and combining Computer Science and Engineering textbooks, instructor-authored sections, and instructor-added notes. The data show that for over 4,000 classes in 2020, over 85% of classes reconfigured their books, over 30% of classes combined two or more books with hundreds combining three or more, about 30% of books had instructor notes added, and about 65% of zyLabs-enabled zyBooks included instructor-created labs. The trend away from static textbooks and toward customizable content has substantial implications on how content is authored, requiring more modularity of content sections to support reconfiguration, and requiring more consistency across subjects to enable combining content. The trend also has substantial implications on book marketing, pricing, renewals, and more.