I have chosen this selection from books I have read over the years.
Beginners:
Abdel Haleem, trans. The Qurʼan (New York: Oxford UP, 2005).
Laurence Brown, The First and Final Commandment, A search for truth in revelation within the Abrahamic religions (Amana publications, 2004).
Martin Lings, Muhammad: his biography based on the earliest sources (Cambridge: Islamic Texts Society, 1986).
Karen Armstrong, Muhammad: A Prophet for Our Time, (Bravo Ltd, 2007).
Bruce Lawrence, The Qur’an: a biography (New York, 2007).
Shaykh Muhammad Al-Yaqoubi, Refuting ISIS (Sacred Knowledge, 2016).
Gai Eaton, Islam and the Destiny of Man, (The Islamic Texts Society, 1997).
Hamza Yusuf, Purification of the Heart, signs, symptoms and cures of the spiritual diseases of the heart, (Starlatch Press 2004).
Intermediate:
Muhammad Asad, The Message of the Quran (new edition, London: The Book Foundation, 2008).
Jonathan A.C. Brown, Hadith: Muhammad’s Legacy in the Medieval and Modern World (Oneworld Publications, 2009).
Jonathan A.C. Brown, Misquoting Muhammad: The Challenge and Choices of Interpreting the Prophet’s Legacy (Oneworld Publications 2015).
Timothy Winter, ed. The Cambridge Companion to Classical Islamic Theology (Cambridge: Cambridge University Press, 2008).
Abdal Hakim Murad, Understanding The Four Madhhabs, (The Muslim Academic Trust, 2012).
Advanced:
Shabbir Akhtar, The Quran and the Secular Mind: A Philosophy of Islam (London: Routledge, 2007).
Muhammad Hashim Kamali, Principles of Islamic Jurisprudence (Cambridge: Islamic Texts Society, 1997).
Abdal Hakim Murad, Bombing without Moonlight: the Origins of Suicidal Terrorism (Bristol: Amal Press, 2008).
Sex and Allah by Christian Prince
Fred Donner, Narratives of Islamic Origins: The Beginnings of Islamic Historical Writing (Darwin Press; 1998)
Fred Donner, Muhammad and the Believers. At the Origins of Islam (Harvard University Press; 2010)
Angelika Neuwirth, The Qur’an and Late Antiquity: A Shared Heritage (Oxford University Press; 2019)