For the SSC CGL mains, both Mathematics and English need a sincere & holistic approach and preparation, as both the papers require a good command of the fundamentals.

Kiran's previous year's questions book is always a good option for practice but might not suffice for the overall preparation.

Assuming you've cleared the prelims and are familiar with the basics, you should now go on to prepare for the advanced-level questions.

Refer to Quantitative Aptitude for CAT by Arun Sharma for mathematics. Continue reading newspapers and magazines to improve accuracy and grammar for the English section. Pay attention to all the concepts and sub-topics.

Once you've reached a point where you can test your preparation, go for Kiran's previous year's question papers and practice as much as you can.