"It is a short book."
Translation:זה ספר קצר.
Apparently זה is more "it" while הוא is grammatically masculine. I don't know if that is a hard and fast rule, but apparently it is a strong tendency at any rate. It kind of makes sense since זה is "this" whereas הוא is "he." My comment below on formal and informal is also something I've heard in DL, but grammatical gender is also a consideration.