If you are interested in the factorization algorithms employed in modern computer algebra systems such as Macsyma, Maple, or Mathematica, then see any of the standard introductions to computer algebra , e.g. Geddes et.al. "Algorithms for Computer Algebra"; Knuth, "TAOCP" v.2; von zur Gathen "Modern Computer Algebra"; Zippel "Effective Polynomial Computation". See also
Kaltofen's surveys on polynomial factorization [116,68,56,7] in his publications list, which contains plenty of theory, history and literature references. Note: Kaltofen's home page appears to be temporarily down so instead see his paper  to get started (see comments)
1 Kaltofen, E. Factorization of Polynomials, pp. 95-113 in:
Computer Algebra, B. Buchberger, R. Loos, G. Collins, editors, Vienna, Austria, (1982).