I just found out that the author J. IJmker (note the capitalisation) gets sorted with the I, as if written Ijmker.
However, in Dutch the IJ is not a combination of the letters I and J, but a single letter
, the 'IJ' (or 'long Y', as opposed to the 'ei' diphthong which sounds the same and is called 'short Y'), lower case 'ij' which is supposed to be sorted with the 'Y' (which is called 'Y-grec' or 'Greek Y' in Dutch).
'ei': Two letters, called 'short Y', gets sorted with E (as expected).
'IJ': single letter
, to be sorted as Y.
Y: called 'Y-grec' or 'Greek Y' and sorted as Y (as expected).
Another small point: Calibre does not recognize Roman numerals in names (as in 'Victor Appleton II' for instance) and also sorts them as 'I'
Can you tell I've been cleaning up my authors starting with 'I' today?