There are no specific books for SLAT preparation. All types of law exams have a similar syllabus.
You need to majorly prepare for the following sections:
1. General Knowledge - Start with reading newspapers, and current affairs
2. Legal Reasoning- Look at previous year papers for an idea about topics like Law of Torts and Law of Contracts, many videos are available on YouTube.
3. Reading Comprehension - Start reading the editorial section of newspapers like The Hindu and The Indian Express
4. Logical Reasoning- The major questions are inference based questions, or reasonings like blood relations and directions.
5. Quantitative Aptitude- Only topics till class 10th come in the exam like percentages, averages etc.