The recordings by Klaas Trapman are really difficult to find indeed. I suppose because the Nederlands Muziekinstituut is not a commercial publisher or editor of CDs. As fas as I know their recordings only can be acquired by ordering at their office.
It was a surprise to me to find out that a lot of the works (and letters) by Bortkiewicz can be found in The Hague, close to where I live.
This information can be found on Wikipedia:
"With much thanks to Hugo van Dalen, his close friend, we can still enjoy Bortkiewicz's music and learn much about his life from the many letters he sent to the Dutch pianist. When van Dalen died in 1967 his family bequeathed the manuscripts of several compositions (such as the 12 Etudes, Op. 29, dedicated to van Dalen); a written autobiography Erinnerungen (published in German in Musik des Ostens, 1971, pp. 136–69, in Dutch by Hugo van Dalen in De Zevende Dag, July–August 1939, and in English by B. N. Thadani, Recollections, 2nd ed., Cantext, 2001); plus a number of letters and printed music to the Gemeentemuseum in The Hague, which recently passed it on to the Netherlands Music Institute (NMI). The NMI has the only existing copy of the manuscript of the Piano Sonata No. 2, Op. 60, and of two of the Preludes, Op. 66."
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