There is a clear stamped letter with daily (floor reset) spawn in several places: for instance in one of the drawers in the Hero Old Fortress here: https://tibiamaps.io/map#33253,31574,9:2A blank stamped letter with a "last edited by ..." note would be something unusual.
As for "why is it empty?", I'd guess lazy streamlined creation of new locations. For the last decade, less and less new books were introduced. Many of them are empty, even when previously we'd have at least a title. The same goes for other writable items, which you "use" hoping to read some lore... but to no avail. Almost nobody reads books nowadays. So having an empty book in a library is at least better than having a library with empty shelves...
Edit: provide exact location
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