• Meaning 1 is most of the time used with other verbs than “lire”, as it is a figurative way to say you do the thing while you haven’t yet finished the book about it. It could be a good thing if it is an activity that requires improvisation and speed, or a bad thing. For example:

  Il prononce son discours à livre ouvert.

  Means that he is giving a talk without having prepared it, just reading one’s text.

• Meaning 2 is usually phrased “lire [en quelqu’un] comme dans un livre ouvert” and means “to read in one’s mind like if it were an open book”.

  Here, as the verb “lire” is used and we are talking about someone’s mind, it seems clear that Meaning 2 is intended.

“à livre ouvert” means that the ideas or the facts are so obvious that you see them directly, with no need of an explanation, as if a book containing the whole description of the situation was opened in your hands.