The diary letters that Philip Slier wrote while being interned in work-camp Molengoot in Hardenberg, were published as Tot ziens in vrij Mokum (Oudewater: Minerva 1999).
More letters of Philip Slier are published in:
D. Slier en I. Shine, Hidden letters, a glimpse of Holland during World War II (New York 2007).
See for more information his biography on Wikipedia.
Presumable this person is commemorated on a memorial in Vierhouten. More information on this memorial can be found (in Dutch) on the website of the Nationaal Comité 4 en 5 mei.