The Portrait of a Lady could be considered a coming-of-age novel in that, over the course of the book, Isabel moves from a naïve and open-hearted young woman—searching for freedom and feeling excited about the prospects of her life—to an embittered adult trapped in a destructive marriage. In this way, James intentionally undermines the literary conventions of Victorian novels (such as those by Jane Austen) that end with joyful engagements or marriages. In the world of James's novel, marriage usually ends up with both parties feeling alienated and trapped.
The Portrait of a Lady is a unique novel in that it straddles the line between Realism and Modernism. James originally published the novel in the early 1880s and then went back and made significant revisions, publishing a new version in 1908.
While the first version of the novel was experimental in certain respects (such as in its cliffhanger of an ending), it was much more in line with the realist literary period. This was because it tried to realistically depict the social conditions and dynamics of American expatriates in Europe in the 1870s, and did so via an observant third-person narrator. The realism still comes through in the ways in which James refuses to romanticize Isabel’s agony over her marriage to Osmond, how he depicts Ralph’s struggles in slowly dying from tuberculosis, and his attunement to the ways art-loving men objectify the women in their lives.
By the time James went back to edit the novel, he was well into the modernist phase of his writing career, meaning he was experimenting much more with typical narrative conventions. His modernist flourishes come across in the ways in which he weaves in and out of Isabel’s mind, abandoning her inner experience for years at a time (including critical years, such as when she decides to marry Osmond and when she loses their child).