If you click on the first of the links below, you will be taken to a page on the enotes study guide that will give you a list of texts that you can turn to if you wish to study this exciting novel further. The majority of these books are all works of literary criticism, so you can expect that they will examine the issues and themes and style of this novel in depth, so they are probably more appropriate if you are studying this novel at college.
However, if you are interested in other works of literature smilar to The Portrait of Lady, you might want to try another novel by Henry James that similarly takes a psychological approach to the discussion and interpretation of character. Two books in particular that I have really enjoyed are The Wings of the Dove, in which a couple hatch a plan to gain them the wealth that they need to marry but are changed as a result through their friendship with the sickly Milly Theale, and then The Ambassadors, where one man is sent by a woman to Europe to bring her son back to safe America. These are both great novels, but The Wings of the Dove is definitely more accessible than The Ambassadors. In addition, you might find some of the author's novellas interesting. Two personal favourites are The Turn of the Screw, a classic ghost story, and The Aspern Papers. In both, the unreliable narrator is used excellently. Hope these suggestions help!