Easy reading on relativity

Kip Thorne's Black Holes and Time Warps (Norton, 1994) is fun and readable. Einstein's own introduction Relativity: The Special and General Theory (Crown, 1961) uses simple algebra (available online). There is an excellent, popular introduction to the Big Bang in Alan H. Guth, The Inflationary Universe: The Quest for a Theory of Cosmic Origins (Vintage, 1998).

General books on the philosophy of space and time

For introductions, Barry Dainton's Time and Space (Acumen, 2001) is excellent, and is a worthy successor to the long-standard, Lawrence Sklar, Space, Time, and Spacetime (University of California Press, 1974). Graham Nerlich's The Shape of Space (Cambridge University Press, 1994) is useful and engaging. For collections of readings, see N. Huggett (ed.), From Zeno to Einstein: Classic Readings (MIT Press, 1999), R. Le Poidevin and M. MacBeath (eds), The Philosophy of Time (Oxford University Press, 1993), and the older J. J. C. Smart (ed.), Problems of Space and Time (Macmillan, 1964). For advanced surveys, the best are Michael Friedman's Foundations of Space-Time Theories (Princeton University Press, 1983) and John Earman's World-Enough and Space-Time (MIT Press, 1989). For books devoted primarily to the metaphysics of time, see Tooley's Time, Tense, and Causation (Oxford University Press, 2000) and W. H. Newton-Smith's The Structure of Time (Routledge, 1980).

General books on the history of theories of space and time

Ancient

R. Sorabji, Time, Creation, and the Continuum (Cornell University Press, 1983).