Quarantine

Author: Greg Egan
Published: 1992
 

This book defied my expectations, which were for a straight-up hard-core SF story. It begins as a typical cyber-punk style mystery and shifts into that hard science vein with quantum physics.
 

It then shifts again into a metaphysical brain-twister which bears comparison to Stanislaw Lem. Alas, it downshifts back through the levels, but not without a satisfactory ending.