Nearly 4 years after releasing their iPad app, Inkling is now available on Android. The new app is still in beta, and it should include many of the same features as the iPad app.
You can find the app in Google Play.
I have to say that I am doubly surprised to see the launch of the app. Given that Inkling has pivoted away from selling textbooks to students, there would seem to be little need for an Android app. Inkling now says that they get most of their revenue from licensing their platform to publishers, so it is possible that they developed the Android app with that in mind.
In any case, if you are looking for a way to spend money on textbooks which cannot be resold, you should check out the app.