The US Government Publishing Office (GPO) has aigitized all Congressional Directories back to the 1800s. They are now available via GovInfo and are useful for committee membership lists, biographies of members and more.
Congressional Directory from the 19th and 20th centuries. Are being digitised by USA GPO. The first release includes Directories from 1869–1888, as well as 1993. Each directory contains lists of parliamentary officials, biographies of members and details on their committee membership.