Links:
[1] https://www.statistics.iisuniv.ac.in/content/e-resources
[2] http://www.iisu.refread.com/
[3] https://ebooks.schandgroup.com/
[4] https://guides.emich.edu/c.php?g=187900&p=1280198
[5] https://www.analyticsvidhya.com/blog/2016/02/free-read-books-statistics-mathematics-data-science/