それは本です。

that (indicating an item or person near the listener, the action of the listener, or something on their mind); it; that time; then; there (indicating a place near the listener); you

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

book; volume; script; this; present; main; head; real; regular; counter for long cylindrical things; counter for films, TV shows, etc.; counter for goals, home runs, etc.; counter for telephone calls

be; is