Computer Science is no more about computers than astronomy is about telescopes.
-- E. W. Dijkstra
Wikipedia's Computer science.
Structure and Interpretation of Computer Programs (by Harold Abelson and Gerald Jay Sussman with Julie Sussman, foreword by Alan J. Perlis).
2 Computability theory
Wikipedia's Computability theory.
An interesting area related to computabilty theory: Exact real arithmetic. For me, it was surprising, how it connected problems in mathematical analysis, arithmetic and computability theory.