See if you have good concepts then you have nothing to worry. Concepts are mainly important. As you are having economical crisis then affording a lot of books isn't possible. Thus I would say if you have internet connection then you can search online for previous years of JEE. These are easily available you can download this papers and practice the problems in order to get idea about it.

Now for Physics there is a book by HCVERMA sir. You can get the soft copy of its both volume online. Download it and just recapitulate the concepts and then start practicing problems. For chemistry emphasize a lot on Inorganic and Organic chemistry. Questions from this portions comes with good weightage. Now mathematics is all about practice, thus if you can afford some practice materials for Mathematics then it would be helpful. You can buy the DPP series by Arihant for practicing problems. See the main funda of JEE MAINS is the problems solving skills. If you build the problem solving skills upto the mark then believe me you will crack it.

For the JEE MAINS revision plan visit the link below:

https://engineering.careers360.com/articles/jee-main-revision-plan

For Previous year question you may visit the link below:

https://engineering.careers360.com/articles/jee-main-question-papers

I hope this answer helps. All the very best for your future endeavors!