Advanced Compiler Design and Implementation
Steven Muchnick's graduate-level compiler text, known as the whale book for its cover and the standard follow-on once the dragon book's front end is behind you. It is almost entirely about optimisation: intermediate representations, data-flow analysis, SSA form, register allocation, loop transformations and interprocedural work, with algorithms given in enough detail to implement. Published 1997 and still the reference many compiler engineers keep on the shelf.