Books in the "Very Short Introduction" series by Oxford University Press

• Christian Art: A Very Short Introduction by Beth Williamson (2004)
• Jesus: A Very Short Introduction by Richard Bauckham (2011)
• The New Testament: A Very Short Introduction by Luke Timothy Johnson (2010)
• The Old Testament: A Very Short Introduction by Michael Coogan (2008)
• The Bible: A Very Short Introduction by John Riches (2000)
• Paul: A Very Short Introduction by E. P. Sanders (2001)
• Classical Mythology: A Very Short Introduction by Helen Morales (2007)
• Myth: A Very Short Introduction by Robert A. Segal (2004)
• Ancient Egypt: A Very Short Introduction by Ian Shaw (2004)
• Ancient Greece: A Very Short Introduction by Paul Cartledge (2011)
• The Roman Empire: A Very Short Introduction by Christopher Kelly (2006)
• The Roman Republic: A Very Short Introduction by David M. Gwynn (2012)
• Late Antiquity: A Very Short Introduction by Gillian Clark (2011)
• The Crusades: A Very Short Introduction by Christopher Tyerman (2006)
• Augustine: A Very Short Introduction by Henry Chadwick (2001)
• Thomas Aquinas: A Very Short Introduction by Fergus Kerr (2009)
• Martin Luther: A Very Short Introduction by Scott H. Hendrix (2010)
• The Reformation: A Very Short Introduction by Peter Marshall (2009)
• Judaism: A Very Short Introduction by Norman Solomon (2000)
• Islam: A Very Short Introduction by Malise Ruthven (2012)
• The Koran: A Very Short Introduction by Michael Cook (2000)
• Islamic History: A Very Short Introduction by Adam J. Silverstein (2010)
• Muhammad: A Very Short Introduction by Jonathan A. C. Brown (2011)
• Buddhism: A Very Short Introduction by Damien Keown (2013)
• Hinduism: A Very Short Introduction by Kim Knott (2000)
• Sikhism: A Very Short Introduction by Eleanor Nesbitt (2005)
• Atheism: A Very Short Introduction by Julian Baggini (2003)
• Agnosticism: A Very Short Introduction by Robin Le Poidevin (2010)
• Humanism: A Very Short Introduction by Steven Law (2011)
• Existentialism: A Very Short Introduction by Thomas Flynn (2006)
• The Meaning of Life: A Very Short Introduction by Terry Eagleton (2008)
• Postmodernism: A Very Short Introduction by Christopher Butler (2003)
• Heidegger: A Very Short Introduction by Michael Inwood (2002)
• Kant: A Very Short Introduction by Roger Scruton (2001)
• Nietzsche: A Very Short Introduction by Michael Tanner (2001)
• Aristotle: A Very Short Introduction by Jonathan Barnes (2001)
• Science and Religion: A Very Short Introduction by Thomas Dixon (2008)
• Protestantism: A Very Short Introduction by Mark A. Noll (2011)
• Catholicism: A Very Short Introduction by Gerald O'Collins (2008)
• Pentecostalism: A Very Short Introduction by William K. Kay (2011)
• Puritanism: A Very Short Introduction by Francis J. Bremer (2009)
• The Quakers: A Very Short Introduction by Pink Dandelion (2008)
• Mormonism: A Very Short Introduction by Richard Lyman Bushman (2008)
• Beauty: A Very Short Introduction by Roger Scruton (2011)

There might be others that would be useful, other philosophers, etc.