The Copyright Office of the Library of Congress has an ongoing proceeding on what to do about orphaned works - those under copyright for which an owner cannot be found at reasonable cost.

I have stuck only a toe into a sea of comments, but there is a lot of interesting stuff, including some actual numbers. For example, the used book site Bookfinder notes:

In 2004, there were at least 29,502 attempts on BookFinder.com to find 13,406 different ISBN-bearing works from such uncontactable publishers. These numbers are based on our most conservative analysis; the actual number of attempts to purchase orphan works is likely between ten and twenty times higher. In most of these cases, no used or new copies of the work were available for sale.

The University of Michigan provides some additional figures on its experience in seeking permissions.

The American Association of Archivists describes one loss to history:

[T]he Eisenhower papers project decided not to print any letters Eisenhower had received. It was not because these letters were not of value in understanding the conduct of World War II, but because “the publishing of documents written by persons still or recently alive presents almost insurmountable legal problems.”

The American Historical Society describes the length to which people must go, unsuccessfully in the cited instance:

The staff’s good-faith effort to track down the current copyright holder of Dumond’s work showcases how difficult, and sometimes futile, such a search can be. Obituaries confirmed that Dr. Dumond passed away in 1976 and that a wife and two children survived him. We then consulted with Dumond’s former colleagues to help locate his relatives and their possible whereabouts (we knew his son was living in Guam in 1976 and his daughter in California, due to searches of older city/state directories—no newer information, however could be found). Contact was made with university libraries, Masonic lodges (to which Dumond belonged), and even veterans’ associations (since Dumond fought in World War I). The AHA staff even contacted the Washtenaw County Probate Office to request a copy of Dumond’s will, in which we discovered that his financial effects had been ceded to a trust company (who would presumably control any royalties generated by the book) that no longer exists. These numerous problems forced us to abandon our project since we could not protect ourselves from infringing upon the possible copyholder’s rights. Despite considerable expense and effort, we were unable to make available work that is only of historical and scholarly, rather than commercial, interest.