“The house of fiction has in short not one window, but a million,” Henry James wrote memorably in his preface to “The Portrait of a Lady.” He added that the imposing structure of his 1881 novel was laid on a “single small cornerstone, the conception of a certain young woman affronting her destiny.” (That “affronting” — as opposed to “confronting” — is Exhibit A for what separates James from most writers.) One hundred and thirty-six years later, Irish novelist John Banville — a consummate stylist himself whose books include the 2005 Man Booker Prize-winning “The Sea” — has dared with his 18th novel, “Mrs. Osmond,” a risky homage and sequel to “The Portrait of a Lady,” to pry open additional windows in the magnificent edifice that many consider the Master’s masterpiece. His goal is not to air out the musty rooms or freshen the place with a contemporary update but to climb in, make himself at home, and suss out what happened next to Isabel Archer Osmond, whose future was left hanging at the end of James’ novel. While the ever-timely themes of women’s liberty and autonomy underpin both books, the two come at these issues from opposite angles. “Portrait” is largely driven by the marriage plot — the question of who this lively, imaginative, fiercely independent young woman will settle down with. But the focus in “Mrs. Osmond” is on a different sort of marital machination: Not who will marry whom, but the many ways in which women in bad unions — including Isabel — can extricate themselves and regain their freedom. It’s almost a given that it’s impossible to live up to the source when taking on a classic. It’s almost a given that it’s impossible to live up to the source when taking on a classic. Another danger is redundancy. While Banville’s many plot recaps assure that readers who haven’t read “Portrait” won’t be utterly lost, “Mrs. Osmond” feels more repetitive than fresh, particularly for those familiar with the original. “Mrs. Osmond” takes up Isabel’s story shortly before James dropped it, two weeks after the death of her beloved cousin and benefactor, Ralph Touchett. Isabel, Banville reminds us, had rushed to Ralph’s deathbed in England from the aptly named