The story of ''Fanny,'' far too much of it true, involves the lives of two Victorian Fannys: Frances Wright, radical advocate of free love, free thinking and (eventual) freedom for slaves; and Frances Trollope, who traveled with Wright to an America she quickly learned to detest deeply and, later, profitably. Edmund White, the well-known author of ''A Boy's Own Story'' and biographer of Genet, here imagines Mrs. Trollope writing a life of her friend, one that reveals almost nothing about Wright and a good deal about Mrs. Trollope.

That's not good news, as the avowed purpose of the novel is to play off one character against the other. But they remain doggedly separate, Wright a confusing enigma whose complex idealism is never explored or explained. She created utopian communities and lectured widely against slavery, the family as a repressive institution and Christianity as absurd, all of which White allows but makes nothing of. Mrs. Trollope is vividly realized and has a knock-you-into-the-third-row story to tell. Trouble is, she told that story very well herself in ''Domestic Manners of the Americans.'' So is there a good reason to read White's vivid retelling when we have Mrs. Trollope's more vivid original? White does add things to ''Domestic Manners,'' sending Mrs. Trollope off on imaginary trips to New Harmony (Robert Owen's planned community in Indiana) and to Haiti. He also invents some sex: an affair between Mrs. Trollope and a freed slave, Cudjo, and a homosexual relationship between Mrs. Trollope's son Henry and the painter Auguste Hervieu, who accompanied her to America.

And he drops in some entertaining cameos. The most extended is of the fatuous Lafayette. ''He preferred to . . . address us,'' Mrs. Trollope says, ''on a general subject: glory. His own.'' Also making appearances, mostly to be funny, are Stendhal, Fenimore Cooper, Jefferson, Irving, Bryant, and Robert and Elizabeth Browning, he robust and iconoclastic, she a simp.

There is also the beginning of a moderately clever postmodern touch: Mrs. Trollope adds comments on her own draft. After some purple prose she says to herself, ''I mean the night was filled with stars -- am I raving?'' She reminds herself to find better terms for ''enormous jiggling bum.'' She writes, ''If men like to take girls' virginity, women lust after men's senectitude'' and then thinks better of it: ''Vulgar, and probably not even true.'' There is, for a while, a commenting ''editor,'' setting up for the first half of the book a kind of ''Pale Fire'' cleverness. But White drops these devices about halfway and leaves his readers with a plain narrative.

That is best when it is actually Mrs. Trollope's. Her trip to the New World, where she found horrid everything from table manners to the baleful effects of equality, is one of the most stirring and amusing of Victorian adventures. Trying to rescue declining family fortunes and, probably, to escape an unbalanced husband, she sailed for America in 1827, inflamed by Wright's vision of the unlimited promise of the wide-open West. Where Wright saw possibilities for human perfection, however, Mrs. Trollope envisioned a way to bilk yokels and accumulate dollars. Settling eventually in Cincinnati, she found modest success as adviser for a museum of marvels, her son posing as ''the Invisible Girl,'' answering any three questions for a reasonable price and using his prep school education in languages to speak in tongues and baffle the locals. Stirred by this success and by alarming financial news from home, she opened a huge bazaar and stocked it with gimcracks from home and useful articles available at a lower cost from shops down the street. It went grandly bust, and Mrs. Trollope returned home and wrote her blockbuster hit, published in 1832, when she was 52 years old. Thus began an astonishing career. She published 113 volumes in 25 years, writing steadily, amiably adjusting her interests to turns in the market. But, though always popular, she never approached the success of her first book.

In that rouser she offered blistering views of everything American. Here's a sample: ''There is always something in the expression or the accent that jars the feelings and shocks the taste.'' ''I never saw an American man walk or stand well.'' Americans are marked by ''overweening complacency and self-esteem.'' ''I do not like them. I do not like their principles, I do not like their manners, I do not like their opinions.''

She often adds little soothers, saying she is simply reporting, leaving ''to abler pens'' an analysis of politics, government and national life. Ha! Her ''chief object,'' she also says, ''is to encourage her countrymen to hold fast by a constitution that ensures the blessings which flow from established habits and solid principles. If they forgo these, they will incur the fearful risk of breaking up their repose by introducing the jarring tumult and universal degradation which invariably follow the wild scheme of placing all power of the state in the hands of the populace.''

An American might say the book ministered shamelessly to British touchiness, a fervent insecurity that needed to hear that where America was similar to England it fell short, and where it differed it erred. In her defense, however, Mark Twain said she was perfectly accurate; and we must remember that her opinions were formed in Cincinnati, an especially ignorant little town that fed on prohibiting things.

The strength of White's book is sucked, vampire style, from Mrs. Trollope's own life and work. White is good when he sticks to mimicking his source. His best prose comes when his Mrs. Trollope is rueful and self-mocking. But he often coarsens her into a comic country gal: she talks boisterously of belching, wind-passing, vomiting, buggering, ''nocturnal congress,'' ''swollen teats'' and roars, ''I might as well lay an egg myself.''

''Fanny'' is strongest when it approaches most closely the zip of the original. When White strays into new ideas, he becomes tediously vulgar. There are some anachronisms: early Victorians did not talk about ''careerists,'' call things ''folkloric,'' use ''attitude'' to mean sassy, say something was ''nuanced'' or use belts with their trousers. Nobody demands such accuracy in this sort of novel; but I think one's enjoyment of it will increase in proportion to one's ignorance of the period and of the two Fannys. So why read it? If you cannot find a copy of ''Domestic Manners,'' it won't positively harm you. It might even stimulate your taste for the real thing. James R. Kincaid, the Aerol Arnold professor of English at the University of Southern California, is the author of ''Annoying the Victorians.''

Copyright 2003 The New York Times Company. Reprinted from The New York Times, Books, of October 19, 2003.