Clover, an inquisitive, loving, fiercely intelligent Boston Brahmin, married at 28 the older and soon-to-be-eminent historian Henry Adams. She thrived in her role as an intimate to political insiders in Gilded Age Washington, where she was valued for her wit and taste by such artistic luminaries as Henry James and H. H. Richardson. Clover so clearly possessed, as one friend wrote, "all she wanted, all this world could give." And yet there is a mystery:...