As you have asked about the books for the preparation of ONGC Graduate Trainee exam for mechanical engineering students, I am giving you information about it, check below for the details
ONGC Graduate Trainee(GT) Syllabus and Exam Pattern:

1.General awareness-50 marks/50 questions
2.Specialization subject-100 marks/100 questions
3.Aptitude test-50 marks/50 questions
Total Time:3 hours
Total number of questions:200
Total Number of marks:200

List of books

Mechanical Engineering Public Sector (Engineer/Management Trainee Recruitment Exam) Mechanical Engineering (HAL, BEL, NHPC, DMRC, UPCL, MTNL, ONGC) Prep Course
Medium: English
Publisher: GK Publications Books

Mechanical Engineers - NTPC/DRDO/RITES/NHPC/IOC/HP/HAL(Executive Trainee, Graduate Trainee, Management Trainee, Officer Trainee, Apprentice Engineers)
Medium: English
Publisher: Ramesh Publishing House Books

Guide to General Aptitude Test (for NTPC, SAIL, BHEL,IOCL, DRDO, GATE, IES)
Medium: English
Publisher: GK Publications Books

A Modern Approach To Verbal & Non-Verbal Reasoning
Author: R. S. Aggarwal
Medium: English
Publisher: S Chand Books