Operating System:
Text Book - "Operating System Concepts" by Siberschatz, Galvin, Gagne
Video Lectures by Prof. V. Kamakoti – IIT Madras [PART 1] [PART 2]
Understanding OS: Linux and Pint OS.
Understanding Multi-threading in JAVA [LINK- Chapter 11, Multithreaded Programming,Java The Complete Reference]

Computer Networks:
Textbook - "Computer Networking: A Top-Down Approach" by Kurose & Ross
Video Lectures by Prof. Sujoy Ghosh – IIT Kharagpur
Linux Networking

Database Management System :
Textbook - "Database System Concepts" By Henry Korth
SQL Queries
Topics like : SQL Architecture (using B Trees), Indexing and Hashing in DBMS
No SQL Database – Document and Graph Type Database like Mongo DB
Complete Understanding of the Database you have created in your DBMS Project.

Data Structures and Algorithms:
Video Lectures on Data Structures by Prof Naveen Garg – IIT Delhi
Video Lectures on Data Structure and Algorithm by Prof. Charles Leiserson and Prof. Erik Demaine - MIT
TextBook – “Data Structures and Algorithms made Easy” by Karumanchi
TextBook – “Cracking the Coding Interview” by Gayle Laakmann Founder and CEO, CareerCup.com
Data Structure Problems from GeeksforGeeks

Programming Practice:
Participate in Programming Competitions and practice Programming Problems from SPOJ , Hacker Rank , Hacker Earth , Codechef or any Other Programming Website.
Learn C++ STL for faster Programming [LINK 1] [LINK 2] [LINK 3]
Gain Experience by participating in "Hiring Challenges" organised by Companies for placements on Websites like HackerEarth etc.
Collection of Topic Wise Tutorials and Practice Problems

Other Stuffs to Study for Placements:
MCQ Questions for Written Test
Object Oriented Design Problems [LINK 1 : Chapter 7 :OOPS Design, Cracking the Coding Interview] [LINK 2]
TextBook - "Logical Reasoning" by R. S. Aggarwal
TextBook - "Quantitative Aptitude" by R. S. Aggarwal
TextBook - "Test Your C Skills" by Yashavant Kanethar
HR Interview preparation
Company Specific Preparation
Brain Teasers