I've got a back button - it's there at the links page when I touch the page number area at the bottom of the screen. And it works, it takes me back to the screen where I was where the super text number was that I touched. Well - takes me back in an ad hoc way ... sometimes I get directly back and other times it dumps me a few pages prior!!
The light symbol shows and then the word 'Back' and over to the right the usual book etc symbols.
Is this the 'Back' button that isn't showing on some people's devices?
Micro SD card in
Epubs (Calibre cleaned and side loaded)
book is in main memory
light is on