Two young men, well-born English cousins, are faced with conflicts between their professional careers and their passion for the arts. One, an ambitious diplomat and amateur connaisseur of the theater, is madly in love with the young muse of the title, whom he has mentored from a gifted nobody into a budding queen of the stage. He wants to have his career and the girl both, but because they are irreconcilable, demands that the girl give up the stage. A true artist, she refuses. He eventually comes around to her counterdemand, but too late, and the novel suggests that his reluctance to "chuck" (as James puts it in his preface) his career for love and art ultimately destroys him, at least psychologically. The second young man -- egged on by a third, a curiously Mephistophelian or Wildean character -- "chucks" career, dependent mother and sisters, and fabulously rich fiancee to become a poor painter of portraits, based on talents he has heretofore barely explored.
I agree with the reviewer who noted that the book is too long for its content. But it also has structural problems that cause it to sag. It lacks a compelling main narrative, so, as I indicated, the reader's attention is divided between two plots whose interconnections are contrived rather than organic. From the title, one expects the young actress to be the leading character, but she is seen mainly from the perspective of her mentor/adorer, who thinks she has no true self - i.e., is always acting. (James gives her a role in the other plot as well, but it is not terribly convincing.) But her determination to ascend the artistic heights IS her main character trait, and she is refreshingly open and cheerful about it. While the conflict between her career and that of her mentor/suitor reaches a high emotional pitch toward the end of the book, he is too thin-blooded to be really tragic, and James keeps distracting us with the other main plot line. The third young man is enigmatic to a fault. He plays a key role in determining the portraitist to follow his artistic dreams, but he comes across as a bad angel. James may have meant him to be some sort of touchstone of artistic freedom, but he is an unpleasant, irresponsible character to hang a career choice on. Finally, James goes to the trouble of giving the portraitist a delightful sister who is not so secretly in love with her cousin the diplomat, only to waste her. She and the actress are the two best characters in the novel, but they take a back seat to the three young men who are respectively indecisive, unbelievably lucky, and unpleasant.
The book's title is not appealing - a "muse" is a female figure important only for her ability to inspire a male artist - but its failure to fulfill the promise of a good tragedy is more disappointing. James at his best is a master of tragedy, usually feminine. The actress is neither a great tragedienne onstage (though James implies she will become one) nor a self-destroying destroyer of men like Wedekind's/Berg's heartless Lulu, which could have made for a more grandly tragic tale. James's actress truly cares about the young man whose heart she breaks - their inability to marry is unfortunate, perhaps, but it is also predictable and does not rise to the level of tragedy. Perhaps James liked and (as the other reviewer said) identified with her too much to let her be cruel. Indeed, by the end of the book it appears that none of the characters is really large enough for tragedy. Ironically, the "tragic muse" is still playing comedy, and her manager's eye is firmly on giving the public what it wants.
I'm in the midst of a quick chronological reading of all James's fiction, and my three stars are based mainly on a comparison with James's own previous works. The Tragic Muse's flaws don't let it measure up to the incomparable Portrait of a Lady, a concise masterpiece like Washington Square, the satirical Bostonians, the strange and touching Princess Casamassima, or even the excellent early novel The American. Still, three-star James is worth reading and rereading. I don't doubt that more will be revealed to me on the next go-round.