I am very sorry and shocked to hear this.
Even though I never got to meet Mark in person, we had been in regular contact for the past fourteen years, during which I had assisted him with translating documents for his ongoing "German Cross in Gold" series, a series which he knew would be his final work. In all of our countless exchanges, I always found him to be pleasant, friendly, considerate and generous.
My payment for the work done was always in books and I have been very happy with this arrangement. Mark had excellent contacts and often went out of his way to find rare titles, out-of-print editions or author-signed copies for me, all this in addition to copies of his own books as well as some others he sent me purely as a gift.
Because of our regular communications, I was of course aware of his ever-declining health.
Mark had been battling with an endless series of illnesses for years, but whenever he mentioned the subject to me, it was quite matter-of-factly and without complaint. In his own words, from a message he sent last November:
"My situation is simply life; no complaints, regrets, or negative thoughts. I have lived TWICE as long as expected since 1st surgery so when first told I never expected to see volume 3 of the German Cross series."
My sincerest condolences to his family and friends.