I seek for the biography of the prophet Muhammad, peace and bless be upon him.
Can you suggest the best book in English?
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There are several of these books. I would advice for a translation of one of the early biographers such as this one (Ibn Ishaq is a fairely respected biographer of the prophet), but the English may seem too heavy. therefore, instead, I would point you to this book, also available in PDF (google search it).
This website actually gives several PDFs (copyright free) and has a comprehensive comparison. This comparison is relevant because the best book for you may not be the best book for someone else: it depends on what you are looking for. For example, you may prefer a book that is more "in phase" with the contemporary world, or a book that is based on the earliest traditions.
Finally, what I would really advice are video lectures. Amr Khaled gave a series, some time ago, called "'Ala Khuta al Habib" (on the footsteps of the beloved). If you can find it in DVD subtitled in English (check carefully if there are subtitles), I would immensely recommend it. He also wrote a book on it, but I don't know if there are translations. But yes, now that I thought about it, it would be my go to choice.