Back in April 2010, I noted that in the past half century Hemingway biographies have largely ignored the papers he left behind at Finca Vigía, his Cuban home, in 1961, and I reported that in 2009 copies of most of these materials had finally become available at the Kennedy Library in Boston, which houses the extensive Ernest Hemingway Collection.
Consolidation of the papers and growing international cooperation among Hemingway scholars have now resulted in a massive project, The Cambridge Edition of the Letters of Ernest Hemingway, to publish his approximately 6,000 letters, of which about 80% have not yet appeared in print. The first volume, covering the years 1907–1922, has already been published; a dozen or more volumes are planned.
The Introduction to the edition by Sandra Spanier, the general editor (Vol. 1, pp. xi-xxxiii), has more on the vagaries of the Hemingway archives. An interview with Spanier is here.