It appears that Apple plans to include support for iBooks Textbooks on the iPhone and the iPod touch with the launch of iOS 7. As noted by 9to5Mac, the iOS 7 App Store includes a new "iBooks Textbooks" section in the educational category.
The section is currently unpopulated, but it does contain the framework for future textbook offerings. There are a number of subject headings, including Humanities, Life Sciences, and Physical Sciences, which include promotional graphics optimized for the iPhone. Section headings are also populated, advertising a number of book bundles like "High School Core Curriculum" and "School Yourself Hands-on Calculus."
The appearance of iBooks Textbooks in the App Store comes following an iBooks Author wording change suggesting Apple could be adding iPhone support for content created with the iBooks Author app. While the wording was later changed back to specify that such content was viewable only on the iPad, the wording has been changed once again to include all iOS devices with iBooks 3.0 or later.
iPhone support for Textbooks and other iBooks Author content is a logical step for Apple as it continues its educational push to get more iOS devices in classrooms. The company has signed several major educational deals in recent months and it has also changed its iTunes Terms and Conditions to allow students under age 13 to create educational iTunes accounts for the first time.
Apple is also planning to expand the reach of its iBooks with Mavericks, introducing an iBooks Mac app for the first time. The app, which is geared toward students, includes a number of study tools like automatically generated note cards and simple note-taking capabilities. iOS 7 is expected to be release shortly after the launch of the new iPhones on September 10, with Mavericks expected somewhat later in October.
Update: The Textbooks section has now been removed from the iOS 7 App Store.