The Library of Congress has digitized E. Milicent Sowerby's five-volume Catalog of the Library of Thomas Jefferson (1952-1959), making the volumes available in either HTML or PDF format. A very useful resource indeed.
[Update: I'm reminded by several people that any use of Sowerby's Catalog should be complemented by a close read of Douglas L. Wilson, "Sowerby Revisited: The Unfinished Catalogue of Thomas Jefferson's Library," The William and Mary Quarterly, 3rd Ser., 41:4 (October, 1984), pp. 615-628.]
