But doesn*t "læse du en god roman" mean exactly that as well? To me, this would translate to "are you reading/Do you read a good novel" and I do not understand why you have to emphasiye the obvious with "er du ved at"... I could understand it if this was something like "I was just reading a good novel when the mailman came" or sth like that. But on its own, it does not make sense.
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) ?