It's a mistake here. The only translation possinble here is: There is a big bool. In Esperanto to give the translation to "It is a big book" it's necessary to put in the beginning of the statement the pronoun "gxi", la formo elektita estas erara, ecx se iuj uzas gxin en konversacia formo.
Can you explain to me why it is wrong? I'm not saying you are wrong I am just fairly new to Esperanto and I want to make sure I understand this concept for later lessons. I am mainly confused because a few lessons back in the 'Tips and Notes' section it said that, for example, you would not say "gxi estas varma nokto" because it is repetitive and you would instead say "estas varma nokto". I probably just overlooked a rule or something so I was wondering if you or anyone could clear it up for me.