"Jest książką" by itself is technically correct, but I'd say, very unlikely, since it's the same as on/ona/ono jest książką, with the pronoun omitted. But it's fine if there is a second noun: "Ten przedmiot jest książką".
Ok, I guess, technically, it works in:
- Czym jest ten przedmiot?
- Jest książką.
But I don't know if I'd ever say that.
As a standalone phrase "Jest książka" is much more likely, since it means "there is a book".
Thanks for the replies. Polish is too hard with all the different endings. It's been about 60 years since I learned about cases. I could not speak English when I started school in grade 1 in Australia, but by the second year I got the hang of it after which it all came naturally. I assume it would be like that for children in all countries. At least my cousins in Poland understand me .