BOOK REVIEW / NOVEL : Star-Crossed Lovers Feed Lincoln Tragedy : HENRY AND CLARA <i> by Thomas Mallon</i> ; Ticknor & Fields $21.95, 362 pages

“Oh, my husband’s blood!” cried Mary Todd Lincoln a moment after John Wilkes Booth fired his pistol into President Abraham Lincoln’s skull. “My husband’s blood!”

But the blood that Mrs. Lincoln beheld was not her husband’s, as we learn in Thomas Mallon’s novel, “Henry and Clara.”

Rather, it belonged to Henry Rathbone, a handsome young Army officer who sustained a near-fatal knife wound as he grappled with Booth in the presidential box at Ford’s Theater.

History has mostly overlooked the wounded officer, but Mallon rescues him from obscurity in a stately and elegant historical novel of classic proportions. As Mallon discovered--and as he tells us in the pages of “Henry and Clara”--the Lincolns shared the state box on that fateful night with Henry Rathbone and Clara Harris, a pair of step-siblings who turned out to be star-crossed lovers.

Henry and Clara first encountered each other when Henry’s mother and Clara’s father, each newly widowed, married and brought together their adolescent children into an early 19th-Century version of the blended family.

It may sound like the familiar premise of more than a few TV sitcoms, but the secret courtship of the step-siblings in the Rathbone-Harris household more nearly resembles the story of “Romeo and Juliet.”

Clara is “streaked with some mischief,” and Henry’s darkly ironic wit “glows like foxfire in a forest,” but their unlikely courtship forces them to contend with the disapproval of their families and the beau monde of power and privilege in which they have been raised.

“Yes, she would have her Lord Byron,” Clara muses over Henry, “but might people now look at Henry as they had looked at the poet, whispering about what he’d done to his sister?”

“Henry and Clara” depicts a raw and roiling American aristocracy in which political connections were an important measure of social status. Indeed, “Henry and Clara” is rooted in both social and political history of the mid-19th Century, and politics seem to matter as much as sexual passion in the lives of these young lovers.

“Politicians don’t make anything,” Henry complains to Clara about the lukewarm political ambitions of her father and the much hotter ones of his mother.

“You’re wrong,” Clara says. “They make history.”

And so it is history that animates “Henry and Clara” and everyone in the book. The looming shadow of the Civil War, the debate over slavery, the election of Lincoln and, finally, the war itself become a strange and deadly backdrop against which the more intimate encounters are played out.

Mallon, for example, allows us to glimpse a sermon by the abolitionist preacher Henry Ward Beecher--and we are invited to regard Beecher as both a pulpit vaudevillian and a moral huckster whose sermons were the equivalent of a movie date for Henry’s and Clara’s generation.

“Passion was passion,” Mallon writes, “and Beecher had it, a lust for drama and catastrophe and redemption that he might have satisfied equally as a stage tragedian.”

Even Lincoln’s presidential campaign turns into a vexing social problem for Clara’s girlfriend Sybil, a belle whose Southern beau is a slaveholder.

“If that abolitionist wins, he’s liable to spoil my whole wedding next May,” pouts Sybil, only half in jest. “A lot of lovely slaves are even part of my dowry.”

Mallon’s book is smart and engaging, and he manages to bring his characters fully alive while never allowing us to forget that they are truly creatures of another era. He makes good use of letters written by the historical Henry and Clara, and he is so steeped in his source materials that his narrative sometimes takes on the grand and florid tones of 19th-Century prose.

“The incompleteness of the Capitol dome made the building feel like a crater,” writes Mallon of the scene of the first Lincoln inaugural, “as if it were inviting the heavens to fling a lightning bolt at this rustic titan who dared to say he would hold together a continent so manifestly wanting to split apart.”

Lincoln, of course, becomes the fateful figure in the lives of Henry and Clara. Thanks to Lincoln, the lightning bolt of history struck them too.

As Mallon eventually reveals, the wounds that Henry and Clara sustained went far deeper than the ones inflicted by Booth’s knife--and we watch as the story spirals downward into madness and much, much worse, as they are finally “crushed by all the hateful history (Henry) worshiped, and in which he’d been caught.”