Undergraduate textbooks
[CLRS] T. Cormen, C. Leiserson, R. Rivest, and C. Stein. Introduction to Algorithms. MIT Press.
[DPV] S. Dasgupta, C. Papadimitriou, and U. Vazirani. Algorithms. McGraw Hill.
[KT] J. Kleinberg and E. Tardos. Algorithm Design. Addison Wesley.
Randomized algorithms
[MR] R. Motwani and P. Raghavan. Randomized Algorithms. Cambridge University Press.
[MU] M. Mitzenmacher and E. Upfal. Probability and Computing. Cambridge University Press.
Approximation algorithms
[V] V. V. Vazirani. Approximation Algorithms. Springer-Verlag.
[WS] D. Williamson and D. B. Shmoys, The Design of Approximation Algorithms. Cambridge University Press.
Online Algorithms
[BE] A. Borodin and R. El-Yaniv. Online Computation and Competitive Analysis. Cambridge University Press.
Network Flows
[AMO] R. Ahuja, T. Magnanti, and J. Orlin. Network Flows: Theory, Algorithms, and Applications. Prentice Hall.
Combinatorial Optimization
[S] A. Schrijver. Combinatorial Optimization: Polyhedra and Efficiency (Vol A, B, C). Springer.