Object Sign in sentences

Why sometimes the sign of object را is omitted in the sentence? Can we always omit it?
For example:
من کتاب می خوانم
من کتاب را می خوانم
من غذا خوردم
من غذا را خوردم

It’s the same as in English, را means “the” in most cases. So it means that it is referring to something already known. You’re examples are perfect. If we say “I read a book” it would be the first one. If we say “I read the book”, then it must be a book that you were talking about beforehand.