One thing to keep in mind when using Last_Modified:
Certain functions can update a book's Last_Modified date without warning or you noticing. For instance, if you found a typo in a Tag (ex: Fictoein) and corrected it in the entire library, then all of the books that make use of that Tag would have their Last_Modified date updated.
This isn't a bug or a problem. Heck, most users would not notice this situation. But you are making active use of this column, so it might happen to you. Just wanted to make you aware of the possibility.