She wouldn’t have many copies of the book, only one each time. But if she knew some kind of secret code, like most of the others do. She could have written it inside the book cover or something like that to where Danial Faraday would have just passed it off as random drawings or something.

She was able to recognize her handwriting from a book which nobody told her anything about. Who’s to say she can’t leave herself some clues.

But beside that, Faraday knows details of Desmonds life and he made it a point to study him and certain events and wrote them down in that book. Desmond is his constant after all. And Eloise from the past now has all of those details about what will happen and can be at certain places to manipulate time.

Once Desmond turned the key however, he can manipulate time because he can see events into the future. As he sees them, he can change them. This is why Eloise was surprised when Desmond decided to actually take the ring. Because the book probably stated that Desmond buys a ring to marry Penelope but then changes his mind. Eloise having read that, made it a point to show up there. However she would have learned that Desmond can now change events because he really wanted to buy the ring. So she quietly learns this fact.

The secret code thing as far as lost goes could not be true, but it IS a possibility. The Others know all kinds of languages and secret codes. If Eloise from the future knew about a code back then. It would be easy for her to on the last page draw a picture with hidden symbols telling herself in great detail about things for instance, and Faraday not knowing anything about that last page, just assume it’s part of the book her mother bought. Just an example, but it would be a way to tell her self something in the past.