This is still a GCSE book....how many years on the trot has this been????
From an english lit point of view it's a great educational tool because you can study what the characters are like in themselves, but also through their relations with other people and the wider society of which they are a part. The characters are also well defined and relatable, and the themes, while interesting, are not subtable.
For that reason it's always been a key part of the GCSE curriculum.