The library button is not visible in DPS apps on iPhone with iOS 11 if the app contains folios beyond a certain length.
Digital Publishing Suite
A change in how iOS handles fonts caused unexpected shifting of user interface elements.
Rebuild the app with release 32.6.8. This release, available in the DPS App Builder as of September 28, 2017, resolves the iOS 11 problem with iPhone viewers where the Library back button does not appear due to long publication names.