1. N L Biggs, E K Lloyd and R J Wilson, Graph Theory 1736-1936 (Oxford, 1986).

Articles:

2. N L Biggs, E K Lloyd and R J Wilson, C S Peirce and De Morgan on the four-colour conjecture, Historia Mathematica 4 (1977), 215-216.
3. G D Birkhoff, Collected mathematical papers Vol. III : The four color problem, miscellaneous papers (New York 1968).
4. L Arias C, A chronological account of the various theories developed for solving the four-color problem (Spanish), Rev. Integr. Temas Mat. 2 (1) (1983), 7-13.
5. D A Holton and S Purcell, The four colour theorem-a short history, Austral. Math. Soc. Gaz. 6 (1) (1979), 11-14.
6. P C Kainen, Is the four color theorem true?, Geombinatorics 3 (2) (1993), 41-56.
7. J Mayer, Le théorème des quatre couleurs : notice historique et apercu technique, in Proceedings of the Seminar on the History of Mathematics 3 (Paris, 1982), 43-62.
8. T L Saaty, The four-color problem : assaults and conquest (New York, 1977).
9. J Wilson, New light on the origin of the four-color conjecture, Historia Mathematica 3 (1976), 329-330.

MacTutor History of Mathematics
[http://www-history.mcs.st-andrews.ac.uk/HistTopics/References/The_four_colour_theorem.html]