"It is a book."
Translation:É um livro.
It's used to mean 'it is' when at the beginning of the sentence and in a context where you've already stated the subject. Ex: "I got you a gift. It is a book." Here the second sentence would translate as 'É um livro' and the 'it' would refer to the gift. You only assume é means 'it is' when used at the beginning of a sentence.
When used in the middle of a sentence, it just means 'is'. Ex: "He is a man." Here the sentence would translate as 'Ele é um homem'. The other difference really is that in the first example a pronoun isn't specified so the default assumption is that you're referring to an 'it'. In the second case the pronoun 'he' IS specified.