Thanks for your fast reply! I am a long time fairly minimalist Fluxbox user so I suspect many variables such as XDG_CURRENT_DESKTOP are unset. I will experiment a little anyway with the ideas from your answer and see if I can get Thunar opening up anyway...
And thanks for Calibre
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Edit: kdialog it is on my system (I have a full Slackware install so despite using Fluxbox there is a full KDE installation sitting by) so I will learn to love it! Most of my additions to Calibre are drag and drop so this is not a major hassle...