Thanks for the replies. I'll look forward to the internal viewer update later.
If you are on page 128 of an atlas and click on a map the internal viewer opens it (no zooming obviously) then when you want to return to page 128 of the atlas you only have recourse to Android's system back button which, when pressed, takes one not back to page 128 but back to page 1 of the book. The Android built in button when in the viewer should be programmed to take on back exactly where they were prior to opening the image otherwise it becomes quite cumbersome to go through an image rich resource.
On the Codex, I hope in a later update that you would increase the resolution of the images. It's a real treat to see zoomed in which I've done with my own shots of the actual codex. Phone screens are very hi res these days and show great detail if it's there.