You don't have the book, you have someone else's interpretation of the book's contents, re-expressed into a language you can read. Both steps can involve a loss or distortion of information, either because the translator doesn't fully grasp the original language or context, or because in the re-expression they chose to leave out details that were relevant to you. The more distant the original language to yours, the more translation involves interpretation, too. A translation is really not too different from a commentary; it's just a different text.