I will give you a few pointers of how this novel relates to a major theory or approach to literature called postcolonialism. It is for this reason that this book is so popular on degree courses as it exemplifies so many of the approaches to postcolonial studies.
Shadow Lines is an amazing story that transgresses multiple borders, one of the key themes of the story. This is also a concern of postcolonial criticism, that examines and criticises man-made boundaries and borders as attempts to define a particular group as against another group ("the other"). Postcolonial criticism attempts to rupture these apparently secure boundaries by examining those who live on the margins of these boundaries and also deconstructing (taking apart) the notion of the other. This is particularly true of the "invention" of India the nation, with the Partition of 1947 which drew imaginary lines across India, creating the countries of Pakistan, Bangladesh and India and also causing much death from the resulting riots.
The narrative in Shadow Lines is constantly transgressing boundaries of space and time, thus giving the novel its title, as the lines that divide places and even times are shown to be easily transgressed - "Shadow Lines."
Consider this quote regarding the inherent fragility of boundaries:
[About seeing the border from the air] But if there aren't any trenches or anything, how are people to know? I mean, where's the difference then? And if there's no difference both sides will be the same; it'll be just like it used to be before, when we used to catch a train in Dhaka and get off in Calcutta the next day . . . (151)
Apart from this key theme you will want to examine the role of women in this novel and also the impact of different generations and their perspectives on issues such as the creation of India. Hope this helps - but again, to reiterate, you have to read this novel for yourself. It is worth it! Enjoy!