That is the exact opposite of what you said you wanted in your first post. You described each workbook having references to a single INDEX workbook. In your last post you describe having a single workbook with references to numerous other ones.
The way to do this with formulas will not work well for you because it will require the use of the INDIRECT function, which requires each referenced external file to be open to work. I imagine that is unworkable with 100 workbooks.
**in Cell A1**
and copy down
The alternative is to use a VBA solution to populate those cells:
However, note that the referenced files should exist first. Each formula will be evaluated by Excel as it is populated into the cell, and if the referenced file does not exist, Excel will prompt to locate the file.