Ignore this if you want no spoilers or your just here to read about her personality and such, not to get book info. If not, you may proceed with no worries.
Dr.Erland stated in the first book ,'Cinder', that his child was a shell and was taken away because his wife was too loyal to the Lunar Queen and thus got killed. Cress, however, was taken away because her mum was to loyal to the queen and was going to get killed but in fact, didnt and was saved due to her hacking skills. I know this is said in the board, but for those who didnt put the peices together imediately, yes, Dr.Erland is her father even though he may not know his 'little crescent moon' is still alive.
But his death was not really very sad, I think because it was like he'd finished his mission in life or something. The scene was really satisfying, for some reason. The only regret I have after his death is that he won't be able to give Cinder any more super-advanced prosthetic limbs. I guess there are other experts for that, though.
In Winter as Dr.Erland is dying he reunites with Cress. It is really not that sad, for me at least, because I was never attached to his character. And Cress, when he dies, feels rather conflicted in her emotions.
spoilers*Dr Erland actually died at the end of the book Cress, not Winter. Also Cress was saved not because of her hacking skills, no shells were killed at all, the shell infanticide laws are just a ruse to use the shells' blood for experiment (letumosis antidote). Then Cress's talents were discovered and was forced to become a spy on earth and locked in a satelite.