For further reading about clocks, see David Landes’ Revolution in Time (Viking, 2000). Longitude (Fourth Estate, 1996) by Dava Sobel is an entertaining history of the discovery by Harrison of clocks accurate enough to be used by sailors to determine longitude.
For the early philosophical history of time, see Richard Sorabji’s Time, Creation and the Continuum (London: Duckworth, 1983).
A good start on the philosophical topics of absolutism, conventionalism and relationalism can be found in Hans Reichenbach’s The Philosophy of Space and Time (NY: Dover, 1958), Lawrence Sklar’s Space, Time and Spacetime (LA: University of California Press, 1974) and Bas van Fraassen’s An Introduction to the Philosophy of Time and Space (NY: Columbia University Press, 1985).
For more on time as the fourth dimension, see George Gamow’s One, Two, Three… Infinity (NY: Dover, 1988) and Rudy Rucker’s Geometry, Relativity and the Fourth Dimension (NY: Dover, 1977).
A completely non-technical but accurate and sophisticated introduction to general relativity is Robert Geroch’s General Relativity from A to B (University of Chicago, 1981).
Some good essays in the philosophy of time are found in Robin LePoidevin and Murray MacBeath’s The Philosophy of Time (NY: Oxford University Press, 1993).
On the direction of time and related issues, see Paul Horwich’s Asymmetries in Time (Cambridge, MA: MIT Press, 1987), Huw Price’s Time’s Arrow and Archimedes’ Point (NY: Oxford University Press, 1996), and Steven Savitt’s collection, Time’s Arrow Today (Cambridge: Cambridge University Press, 1995).
Roger Penrose’s discussion of the Second Law of Thermodynamics in The Emperor’s New Mind (Oxford: Oxford University Press, 1989) is recommended.
For time travel, Paul Nahin’s book, Time Machines (NY: Springer-Verlag, 1999, 2nd ed.), is a must. The book is an entertaining discussion of time travel as it appears in physics, metaphysics and science fiction, and it contains an almost exhaustive bibliography. The books by Horwich and Savitt also contain good discussion of time travel.
And for the implications quantum gravity may have for time, see Julian Barbour’s The End of Time (London: Phoenix Paperbacks, 1999) and Craig Callender and Nick Huggett’s collection, Physics Meets Philosophy at the Planck Scale (NY: Cambridge University Press, 2001).
Good Internet resources include the entry on Time in the Internet Encyclopedia of Philosophy (www.utm.edu/research/iep/t/time.htm) and the entry on time travel and modern physics in the Stanford Encyclopedia of Philosophy (plato.stanford.edu/entries/time-travel-phys).