There are three levels of preparation for the CAT exam and the material required for each of them will be different.
The three levels are basic level, concept level and CAT level.

Basic level is specially for students who find it difficult to cope with the subject. Students who are comfortable with the subject need not refer to this. Good notes for this level are school books, banking exam notes or notes for speed based exams like MBA CET. The sums out here are simple to understand and will help in basic understanding of the topics. One of the author that can be referred out here to s R S Agarwal.

Concept level. Here the assumption is that you are fine with the basics and are looking at solving tougher sums to clear your concepts. Here you can refer CAT notes of any classes or books on CAT by Sarvesh verma, Arun Sharma, Nishit Sinha, etc. The questions that are available are usually both easy and difficult.

CAT level, I would say is different from concept level in terms of the type of questions rather than on difficulty level. Most of the time the sums that come in the exam are quite different from what is there in CAT books, though the concept level books will also have some questions of CAT type. The best source of these is the past CAT papers and mock papers, both present and past, of various classes. These questions are based on what have come in the past in CAT exams or the types that can be expected in the CAT.

You can download some of the CAT books from our official website.. https://bschool.careers360.com/download/cat-ebooks

Best wishes. Thank-you.