The CDAC Common Admission Test has three sections which are as following:

SECTION - A
English, Logical Reasoning and Quantitative Aptitude.

SECTION - B
C Programming, OOP and Operating Systems, Data Structures, Data Communications and Network.

SECTION - C
Computer Architecture, Digital Electronics and Microprocessors.

Following are some tips which will help you in cracking this examination:

Understand the complete syllabus
Solve previous year question paper
Use standard books.

For your reference here I am telling you the names of some good books for the preparation of this examination:

Digital Logic and Computer Design
Morris Mano
PHI

Microprocessors and Interfacing
Douglas V Hall
TMH

Data & Computer Communication
William Stallings
PHI

Computer Organization and Architecture
William Stallings
PHI

The C programming Language
B W Kernighan & Dennis Ritchie
PHI

Data Structures
Lipschutz and Seymour
Schaum Series

Operating System Concepts
Silbeschatz and Galvin
John Wiley & Sons