GreenAir
Each reading application has an ability to notify Sony shell about the book it has open, the place (page) you are reading, etc. This what will become visible in the Sony shell as "Continue reading" information after reading application closed (moved to the background). So application needs to be modified a bit. For example over the time CoolReader and AlReader applications have been modified to work properly with Sony shells on T1 and T2 devices. I posted necessary code long time ago
here.