IELTS exam is a level-based proficiency test that allows universities & countries to know whether your English language is strong, moderate, or weak. Many books & resources are available, but you must have a solid base to get a better band.
You can access grammar books, exercises, and rules from the internet. A firm hold over your grammar will allow you to concentrate on other factors, as this is the initial requirement.
Grammatical skills
Free material like English movies and podcasts are available for you to work on your flow and vocabulary. Movies are a great source of native-like expressions.
Movie and podcasts
The Cambridge books are available in the market, but when you register for an IELTS exam, you get a free Cambridge practice book as preparation material before the exam.
Books
Many videos on YouTube and listening transcripts are free for you to explore and know more about IELTS.
Videos, scrips, and transcripts
Your research related to the material must be both language and format oriented. The organization and structure of an answer will depend on your research.