If you use vocab flashcards that contain an example sentence or two showing usage in context, then it is more efficient than reading a book (with regard to the language rather than the book's actual content). Personally, I read books and when I encounter an unfamiliar word, I add it to the flashcard pile, ensuring that I don't forget the new knowledge.

If you're trying to learn the content of the book, the process of converting the information into flashcards entails engaging with the material, so that isn't skipped.