Why in this case you can omit the subject in esperanto?
Should it not be libron?
No, "esti" doesn't use -n with its object.
Esti is to be seen as "=". What follows describes (adjective) or precises (noun group) the subject. -N shows the aim of an action or a motion. Other frequently used verbs which doesn't take the accusative: ŝajni (to seem), fariĝi aŭ iĝi (to become).