Should be simple bash.
Code:
for i in /mnt/us/documents/*.sdr; do
if [[ ! -f "${i%.sdr}.azw3" ]] && [[ ! -f "${i%.sdr}.azw" ]] && \
[[ ! -f "${i%.sdr}.mobi" ]] && [[ ! -f "${i%.sdr}.kfx" ]]; then
rm -rf "$i"
fi
done
...
Though really, it doesn't matter as they aren't very big and won't save much space at all.
