Hii Sumit, there are many posts under CAPF, and the recruitment process for each post is different .you have to first choose for which post or exam your are going to prepare for.

Exams and posts under CAPF.

1- SSC GD - staff selection commission each year releases notification for recruitment of constable general duty. 

How to prepare for SSC GD

You can start your preparation simply by taking any previous year question paper booklet.because in SSC gd questions are asked from 9th & 10 th standard.

Just buy any previous year question paper booklet.  practice it . Take help of any  online platforms. That will be sufficient for cracking SSC GD.

2- SSC CPO- for getting a post of sub inspector in CAPF you have to follow these books and strategies.

How to prepare for SSC CPO

For English- you can take help of plinth to paramount.

For reasoning- you can take Kiran previous papers.

For maths - Rakesh Yadav class notes & Kiran previous year.

For GS/GK- study lucent GK .and follow current affairs by online platforms.

Give as well mock test as you can, it will boost your marks .

3- CAPF ( AC) - this examination is conducted by UPSC every year and very few seats are available for .so follow these books and strategies to get a seat.

How to prepare for CAPF(ac) 

There are two papers for getting selection.

Paper 1 - general ability and intelligence 250  marks. For maths , English , reasoning,and current affairs with above mentioned sources you have to study NCERT of 11 th & 12 th for history , political science, economics,and geography.for general science you can take lucent general science book .

Paper 2- General studies,essay and comprehension. For this paper prepare modern history, especially freedom struggle part write it down on paper and review it again and again.for English and other parts of the paper stay updated with current affairs and figure out the topic which is of national importance , Practice these topics with pen and paper .that's all best of luck