There are quite a few online coaching centers like IMSIndia, Career Launcher, Gradeup, AlchemistIndia, Time4 Education,etc. But these are not free. People upload tips on Youtube but we cannot vouch for their credibility and success. But you can as well learn and prepare for CAT from the comfort of your home. Here are few books:

CAT Books For Quantitative Ability:
1.NCERT mathematics books (from 6th to 10th class) ::
These books are a good way to start if one needs to clear their basics and concepts. NCERT books are best to start your CAT Preparation with only if you have a lot of time before your exam date and need to brush up on concepts before solving advanced problems.

2.How to Prepare for Quantitative Aptitude for the CAT (by Arun Sharma)
(publisher: Tata McGraw Hill)
This is a very popular book among CAT aspirants and really easy and simple to understand. This book also focuses on the basics first and has different levels of difficulties step by step.

3.Quantitative Aptitude for Competitive Examinations (by Abhijit Guha)
This book is quite comprehensive for various competitive exams, for both MBA aspirants and for those seeking jobs who need to prepare for the aptitude tests.
4.Quantitative Aptitude Quantum CAT by Sarvesh Verma
It is an extensive book for students aspiring to go to IIMs. It has a very simple, lucid and tailor made content revolving around CAT.

5.Quantitative Aptitude for CAT (by Nishit Sinha)
This book by Pearson focuses on in depth understanding of core topics for basic and advanced applications.


Verbal Ability Books For CAT:

1.How to Prepare for Verbal Ability and Reading Comprehension for the CAT( by Arun Sharma and Meenakshi Upadhyay)
This book is good to brush up your verbal and reading comprehension skills. It covers three important topics: reading comprehension, verbal ability and verbal reasoning. It provides sufficient practice and mock papers for self-assessment.

2.Verbal Ability and Reading Comprehension For The CAT( by Nishit Sinha)
This book by Pearson provides not only an exhaustive resource for verbal ability. It also includes numerous tests as well as integral problem solving techniques and approaches.

3.Word Power Made Easy (by Norman Lewis)
This is the go-to book to improve your vocabulary. It is not a book to be read but a book to work with daily to enrich your vocabulary.

4.High School Grammar and Composition( by Wren and Martin)
It is a very good book for people who want to improve their grammar skills. It helps to understand and conceptualise different aspects of grammar and it is a must have book for anyone with poor grasp on verbal ability or who wants to make up with the basics of english.

Read books from different genres
The ultimate key to improve your verbal ability is to read extensively and to read books from different genres. This ensures that when the time for reading comprehension comes you are well acquainted with different writing styles and topics. Reading extensively also improves your reading speeding over a period of time. There are no shortcuts here.

Data Interpretation and Logical Reasoning Books For CAT:

1.A Modern Approach to Logical Reasoning( by R.S. Agarwal)
This CAT book by R.S. Agarwal is to start with the most basic reasoning problem as it is quite simple to understand. It is segregated into different sections such as logic, statements, inferences and arguments, assumptions, conclusions, course of actions, logical puzzles, etc.

2.How to Prepare for Logical Reasoning for CAT book (by Arun Sharma)
This book by Tata McGraw Hill contains all topics to be considered under Logical Reasoning for CAT as well as other entrance exams. Each topic is explained with solved examples followed by a series of problems with varied difficulty levels to solve which gives an individual enough practice for this section.

3.How to Prepare for Data Interpretation for CAT book (by Arun Sharma)
This book focuses on all areas of Data Interpretation and Data Sufficiency. It provides and emphasizes on developing our calculations to get an edge with DI. This book also provides you with previous years questions, Model and Sample test papers.

4.Logical Reasoning and Data Interpretation for the CAT (by Nishit Sinha)
This book covers fundamental and advanced topics under the aforementioned section which are explained with the help of solved examples, case lets and exercises to do. It is useful for various other exams as well.

5.Data Interpretation (by Nishit Sinha)
Practice Book for CAT and Other MBA Entrance Examinations( by Nishit Sinha):
This book provides adequate practice and to sharpen the skills for Data Interpretation. It is also quite useful for other competitive exams like NMAT, GMAT, SNAP, XAT, CMAT, etc.

Another CAT Preparation material or book to be considered is Previous Years’ Solved CAT Paper:
This ensures that you are familiar with the kind of questions that are asked in CAT providing you an edge. They can be taken as mocks and the analysis of the same can provide you with great insights for your preparation.

It is always better to read newspaper for CURRENT G.K. than trying to grasp all info at once. Some of the newspaper sources which you can use to prepare this section are The Hindu, Financial Express, and The Economist.

In case, you don’t like reading newspaper daily, you can go with Manorama Yearbook or Current Affairs Yearly by Arihant for General Knowledge which is published every year.

In order to prepare for STATIC G.K. section, you need to broaden your overall scope in following sections: Sports, Politics, Entertainment, Biographies, etc. So, for this, you can use Static General Knowledge by A.P. Bhardwaj and S. Chand’s Advanced Objective General Knowledge. These books are okay to start with if you are not someone with a newspaper reading habit from the past.

Hey!

you can find tonnes of videos about cat and its preparation on youtube. most of them are helpful. you can choose the channel on your own choice of language and comfort.

regarding classes. there are few websites like udemy, gradeup, careerlauncher that do courses on CAT. but they are not free. they are little cheap compared to institutions. but still not free.

the free ones are not that good. so use youtube and the above websites.