I can recommend the book Computational Complexity Theory,
edited by Steven Rudich and Avi Wigderson, which is based on a graduate summer school by the IAS/Park city mathematics institute, with lectures by various experts in the field.
The purpose of this book is to provide the basics, some history and a glimpse into the research in some areas of computational complexity theory, aimed at mathematics students.
As such, this book is not a replacement for an introductory book such as Sisper's, but can be read independently to get a feeling what some sub-fields of complexity theory look like, and can help to guide a future focus.
For completeness, I'll list a summary of the table of contents as well (see this Google books preview for the full table):
Week One:
COMPLEXITY THEORY: FROM GÖDEL TO FEYNMAN
Steven Rudich, Complexity Theory: from Gödel to Feynmann
Avi Wigderson, Average case Complexity
Sanjeev Arora, Exploring Complexity through Reductions
Ran Raz, Quantum Computation
Week Two:
LOWER BOUNDS
Ran Raz, Circuit and Communication Complexity
Paul Beame, Proof Complexity
Week Three:
RANDOMNESS IN COMPUTATION
Oded Goldreich, Pseudorandomness - Part I
Luca Trevisan, Pseudorandomness - Part II
Sadil Vadhan, Probabilistic Proof Systems - Part I
Madhu Sudan, Probablistically Checkable Proofs