hi, i'm sorry to hear about your dad. but i hope that he will be feeling fine soon. my dad was diagnosed with cancer 7 years ago and he weighed about 110lb during radiation treatment. he has slowly gained 10lb back over the years. i'm not sure how tall your dad is, but my dad is 5'4' so his weight right now is not bad at all.
there is always a reason for drastic weight loss - did your dad check out what happen recently for his recent weight loss? i'd suggest to double check with the doctor first.
is he planning to gain his normal weight back? is he a vegetarian? is that why you are planning to buy a vegetarian cookbook? it sounds like you are doing the right thing by encouraging small frequent meals!! i found an article here which my dad constantly referred to when he was trying to gain his healthy weight back. once he reached his healthy weight, he started to eat healthy again to maintain his weight. hope it helps!!!
http://www.healthcastle.com/cancer-weightloss.shtml