What should I start reading if I want to get serious with quantum information theory?
The answer to this question depends on your background. You will need a basic understanding of (abstract) quantum mechanics, theoretical computer science and information theory.
The following books and articles have proven to be very useful for just that, and give the kind of information that is useful for the field.

• Quantum Physics:
  • `Quantum Theory: Concepts and Methods' by Asher Peres [book], Fundamental Theories of Physics, Volume 75, Kluwer Academic Publishers (1993).
  • `The Feynman Lectures on Physics, Volume 3' by Richard P. Feynman [book], Addison Wesley Longman, Inc. (1985)
• Books/Tutorials on Quantum Computation:
  • "Introduction to Quantum Computation and Information", edited by Lo, Popescu, Spiller (World Scientific, 1998)
  • Preskill's Lecture Notes. The notes of John Preskill's course on quantum computation at Caltech (1998).
  • Andre Berthiaume's introduction to quantum computation. Also on the web (1996).
  • A review article by Andrew Steane on the Los Alamos archive (1997).
• Complexity Theory:
  • `Computational Complexity' by Christos H. Papadimitriou [book], Addison Wesley Longman, Inc. (1993).
  • `Fundamentals of Algorithmics' by Gilles Brassard and Paul Bratley [book], Prentice Hall (1995).
  • `The Feynman Lectures on Computation' by Richard P. Feynman, Anthony J.G. Hey and Robin W. Allen [book], Addison Wesley Longman, Inc. (1996).
  • `Introduction to Algorithms' by Thomas H. Cormen, Ronald L. Rivest and Charles E. Leiserson [book], MIT Press (1990).
• Cryptography:
  • `Codes and Cryptography' by Dominic Welsh [book], Clarendon Press, Oxford (1988).
  • `A Bibliography of Quantum Cryptography' by Gilles Brassard (1994-1998).