Informative nonfiction for summer reading; Slavery's toll -- on the masters; The Ruling Race: A History of American Slaveholders, by James Oakes. New York: Alfred A. Knopf. 307 pp. $16.95.

Anyone who visits the plantation home of Thomas Jefferson is likely to wonder: How could our Southern forebears embrace a document proclaiming ''all men are created equal'' -- while they carried a slaveholder's whip?

This paradox, according to historian James Oakes, reflected a deep-seated conflict of values in the Old South. On one hand, the hierarchical, feudal model that had traditionally justified slavery was giving way to the egalitarian ideal fostered by a market economy. But meanwhile, ironically, these same market forces made slaveholding too profitable for white Southerners to resist.

Thus, Oakes contends that although most slaveholders never questioned their supposed racial superiority, they frequently suffered private guilt -- along with fear that in the afterlife, ''while death liberated the slave, it doomed the master.''

The tortuous, self-contradictory arguments Southerners conjured up in defense of slaveholding are a focal point of ''The Ruling Race,'' which may be the first comprehensive study of American slaveholders from Colonial times to the Civil War. As Oakes notes, scholarly works on slavery have abounded since the 1960s, but their emphasis has almost always been on the slaves, not their masters.

Oakes makes careful use of demographic evidence as well as diaries and letters of the period to draw a vivid picture of the antebellum South -- one that differs sharply from the ''Gone With the Wind'' stereotype often accepted by its denigrators and idyllists alike. For the most part, the author avoids editorial judgment of slavery, allowing slaveholders to speak for themselves. And that may be the most effective condemnation of all.