Depends on your definition of practically unknown. With that said, these are the four that immediately spring to mind as being both worth reading and relatively obscure (judging by date of publication in conjunction with being either out of print or with very few star ratings on Amazon).

Did Monetary Forces Cause the Great Depression? - Peter Termin, 1975

Termin is still going strong at MIT. His 1975 book was foundational for challenging Friedman on the cause of the Great Depression. Given what was to come in the 1980s this book quickly became overshadowed and destined for obscurity. However, it still provides an appropriate, timely lens to analyze monetary theory without the abstraction that has engrossed economics as of late.

The Supreme Court in the American System of Government - Robert Jackson, 1955

A series of lectures created for a Harvard lecture series in 1954-55 by Justice Jackson. He suddenly died before being able to deliver them, but they were compiled in a book now out of print. Justice Jackson is widely regarded - across the aisle - as one of the most brilliant legal writers of our time (or perhaps of any time). While this book doesn't set out his entire judicial philosophy, or even do what the title says due to his untimely death, it does lay a valuable conception of the proper role of the SCOTUS within the Republic. Also recommended, to see both his pen and intellect in action, are his opinions in Korematsu v. United States and West Virginia State Board of Education v. Barnette.

The Opium of the Intellectuals - Aron, 1955

Amazon does a better job of summarizing than I could off the top of my head, so here you go: "Raymond Aron's 1955 masterpiece The Opium of the Intellectuals, is one of the great works of twentieth- century political reflection. Aron shows how noble ideas can slide into the tyranny of "secular religion" and emphasizes how political thought has the profound responsibility of telling the truth about social and political reality-in all its mundane imperfections and tragic complexities."

An incredibly difficult read that is worth trying to get through. Brimming with ideas and not without its own pitfalls. Tells the story of 20th Century intellectual history and thought as well as any could, although in a rather indirect way.

The Protestant Establishment: Aristocracy and Caste in America (Aristocracy & Caste in America) - Baltzell, 1987

I'll let Amazon summarize again: "This classic account of the traditional upper class in America traces its origins, lifestyles, and political and social attitudes from the time of Theodore Roosevelt to that of John F. Kennedy. Sociologist E. Digby Baltzell describes the problems of exclusion and prejudice within the community of white Anglo-Saxon Protestants (or WASPs, an acronym he coined) and predicts with amazing accuracy what will happen when this inbred group is forced to share privilege and power with talented members of minority groups."

My summary would be: what will happen (hypothetically, remember the date of publication) when an ephemeral class (WASPs) suddenly disappear from their previous pedestal of influence? Prescient, widely applicable to other countries with their own quasi-classes, and deeply interesting for those less familiar with the subject.