For Class 6 Sainik School entrance exam, there are specific books that can help you prepare well. You should focus on Mathematics, English, General Knowledge, and Intelligence. NCERT books of Classes 5 and 6 are a good base, along with practice guides made especially for Sainik School exams. I will be attaching the link so you can check the recommended book list in detail.
https://school.careers360.com/articles/aissee-books