Middlemarch was on my A level English text and I can remember at 16 thinking what an unsuitable book it was for 16 - 18 year olds. It is primarily a study of marriages from Dorothea and Casaubon, her sister's marriage, Lydgate and Rosamund Vincy, the elder Vincys, Bulstrodes and several others. At 16 I was aware that I did not have enough experience to understand the book other than in a most superficial way.
Twenty years later I came back to it and was pleased to discover my teenage judgement was correct. As an adult it is a riveting read, the subtle interplay of how a range of different marriages work, the balance of power within them and the way individuals in a couple will manipulate their partner to achieve their aims was spell binding.