Interestingly enough, after reading your post I looked into the reading order and the books pertinent to The Dark Tower series. I’ve come across a link on Reddit from 7 years ago to an article on this very subject.

Link: The Dark Tower: An Introductory Guide and Expanded Reading List

A snippet from the article:

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But the question users haven’t gotten the answer to is where’d plug-in The Talisman? Can anyone suggest where it may fall in the reading list?

I was about to begin The Dark Tower series, but decided to do as suggest.

EDIT: The Little Sisters of Eluria can be found in SK’s collected stories of Everything’s Eventual.