Following a sudden disappearance, the Google Books iOS app has returned to the App Store with an update, v18.104.22.16883. Google's release notes claim only that "minor enhancements and bug fixes" have been made. In reality, the focus appears to have been stripping a button for Google's eBookstore.
Several other prominent apps have also had store links stripped. These include Kobo, Rhapsody, Amazon's Kindle, Barnes & Noble's Nook and the Wall Street Journal.
Under the terms of Apple's subscription rules, apps are allowed to show or play content purchased externally "as long as there is no button or external link in the app to purchase the approved content." Apple prefers that sales be handled in-app, where it can claim a 30 percent cut of revenue. Regardless, the company held off on enforcing the rules for some time, particularly for apps from high-profile developers. A sudden rush of crippled updates may suggest the passing of a quiet deadline.