I think that "Niska", and its derivatives, only apply to shortness upwards from the ground: people and buildings, for example. So that's "short" as opposed to "tall".
OTOH, books are "Krótki" meaning "short" as opposed to "long".
Isn't it "Эта книга" ("Eta kniga") or is "żk" a fancy transliteration for "g"?
It's книга officially, but книжка is the very common diminutive alternative used by most people. Книга sounds too formal in many contexts.
I think so, but I'd feel the urge to point individually at the two books being discussed.