"They are going to publish their book at half past eleven."
Translation:Họ sẽ xuất bản cuốn sách của mình lúc mười một giờ rưỡi.
As far as I'm concerned, you can only omit it for something you only have one (country, body parts, etc) or for people (e.g. father, teacher, friend, etc). Book isn't one.
In the English sentence, one nuance of it is "They are going to publish their book, not the speaker's (or the hearer's) own", not as stressed at the possessive word "của" as in the Vietnamese sentence one. But still, it is not possible to leave the word "của" out.