how do i get books to use for my online library service

HI
I am starting a new business which is similar to the kindle unlimited book service. My question is how do i get the books for my library as in do i need permission or do i just buy the books and inform the publisher, what consent do i need and who do i get it from. I know it sound like the basics i would have to know but this is proving the most difficult to find out any help would be very much appreciated
Thank you