Here are some additional categories of "orphan work" authors for whom we have found leads through rudimentary online searching and through your comments.  (And, yes, we know, we should have identified Sidney Hook much earlier.)

Thanks again for all the help. Please go to Orphan Row to help us find others.

Sidney Hook

American Philosophers at Work (1956)

Status: Rightsholder Found!

Direct descendants living in California.

Authors with awards in their honor

(this should have been gathered with others in last post)

Fletcher Pratt

Stanton, Lincoln's Secretary of War (1953)

Status: Weak lead

Mr. Pratt served as president of Civil War Roundtable in New York, which sponsors an award in his name.

Authors with papers collected at academic or other archive

James Mark Baldwin

Between Two Wars, 1861-1921, (1926)

Status: Strong lead

Papers reside at Princeton.

Cyril Bibby

How Life is Handed On (1947)

Status: Strong lead

Papers reside at the Cambridge University Library.

Dana Burnet

Four Walls (1928)

Status: Strong lead

Papers reside at the University of Oregon.

Carroll Lane Fenton

Prehistoric world (1954)

Status: Strong lead

Papers reside at the University of Iowa.

Denna Frank Fleming

The United States and the League of Nations (1932)

Status: Strong lead

Papers reside at the Jean and Alexander Heard Library, Vanderbilt University.

Francis Stuart Harmon

The Command is Forward (1944)

Status: Strong lead

Papers reside at the University of Minnesota, Kautz Family YMCA Archives. Mr. Harmon was on the Board of Deacons of Riverside Community Church in New York and a trustee of the Interchurch Center until the late 1960s.

Hornell Norris Hart

Personality and the Family (1941)

Status: Weak lead

Some papers reside at Swarthmore College, though the college notes that it is not the official repository for his papers.

John Tasker Howard

Stephen Foster, America's Troubadour (1939)

Status: Strong lead

Mr. Howard was a curator of the New York Public Library from 1940-1956.  The John Tasker Howard papers, 1869-1995, are held at the library.

Jacob Lestschinsky

Dos sovetishe Idntum (1941)

Status: Strong lead

According to JewishVirtualLibrary.org, “His collection of books and papers, which he somehow maintained throughout his wanderings, are at the Institute of Contemporary Jewry of the Hebrew University.”

Lloyd Lewis

Myths After Lincoln (1941)

Status: Strong lead

Papers reside at the Newberry Library in Chicago.

Henry Justin Smith

It's the Way It's Written (1923)

Status: Strong lead

Papers are at the Newberry Library in Chicago.

Authors with identified direct descendants

William Edgar Borah

American Problems (1924)

Status: Strong lead

Direct descendant in Washington, DC. (Thanks, Ira Stoll!)

Axel Brett

A Critical Approach to an Esthetic Theory (1926)

Status: Strong lead

Direct descendant located in Texas.

Winfred Octavus Bryson

Negro Life Insurance Companies (1948)

Status: Strong lead

Direct descendant located in Ohio.

Max Loehr

Buddhist Thought and Imagery (1961)

Status: Strong lead

Author’s sons live in South Carolina and Washington State.

Sándor Lorand

Technique of Psychoanalytic Therapy (1946)

Status: Strong lead

Strong evidence that author’s widow is alive.

Richard Pattee

The Case of Cardinal Aloysius Stepinac (1953)

Status: Weak lead

Direct descendant living in Canada.

José Pijoán

An Outline History of Art (1938)

Status: Strong lead

Direct descendant located in New Mexico.

E. R. Root

Lecture on Bees (1925)

Status: Weak lead

Direct descendant located in Ohio. (Thanks, marshallbrooks!)

Authors with potentially identified direct descendants

Irving Lorge

Psychology of Adults (1963)

Status: Weak lead

Appears to have living direct descendants.

William Sansom

South, Aspects and Images from Corsica, Italy, and Southern France (1950)

Status: Weak lead

Direct descendants may be living in the U.K.

Elizabeth Hough Sechrist

Heigh-ho for Halloween! (1948)

Status: Weak lead

Obituary gives names of surviving sister and niece.

Author who worked in U.S. publishing industry

Fon W. Boardman

Roads (1958)

Status: Weak lead

Author was vice president of Oxford University Press in New York, according to correspondence at the Merton Center.

Removed from Orphan Row by HathiTrust:

James Branch Cabell

The High Place (1923)

Status: Presumed Found

Pulled from Orphan Row by HathiTrust. Might be public domain.

And finally…

Richard Gerstell

How to Survive an Atomic Bomb (1950)

Status: Weak lead

Author was Director of Civil Defense for Pennsylvania in the 1960s. Has presentation digitized in National Defense University Library.