When I started reading in horizontal mode I realized this is a convenient layout: It allows for the next and prev buttons to be positioned directly in the center under the thumb. And the back and confirm buttons, separated on different sides, also feel convenient. The device is balanced by its weight and requires no movement while reading.

Also, other conventional variations are already implemented:

1. Left Right Back Ok
2. Back Ok Left Right
3. Left Back Ok Right

The only one missing is this one: Back, Left, Right, Confirm