If you are looking for Operating System Book for knowledge purpose and to clear GATE Exam then Galvin is sufficient. In Gate Exam Questions from the following topics are generally asked in GATE exam.
(1) Process Concepts
(2) CPU Scheduling Algorithms ( 2 Marks Every Year Asked)
(3) Banker’s Algorithms
(4) Memory Management Questions like Calculation of Average Access time, Page replacement algorithms based questions, Logical address and physical address based questions.
(5) Questions based on semaphore or critical section
(6) Disk Scheduling Problems
You can read Operating System Tutorials article to know more about Operating System for GATE exam