Actually, as I have learn, "sider og læser" is more a matter of saying about you reading while sitting. It therefore implies a state of mind and so on.
Now I would do like that :
Are you reading a good novel? = Læser du en god roman ?
Er du ved at læse en god roman? = are you going to read a good novel ?
Sider du og læser en roman? = Are you reading a novel, well sitted (in an armchair, the cat on your knees with a coffee mug aside) ?