The mekk.calibre package contains a helper scripts for some Calibre tasks.
The following scripts are available:
Checks books without ISBN (set in metadata) for ISBN-like string present in leading pages. If found, add it to the metadata (what makes it possible to download full metadata, covers, etc).
Convert any .doc to .rtf (unless already present) - using openoffice.
Checks given directory tree for books not yet present in calibre, add them if found. Uses binary file comparison to check whether the file is identical (file name and metadata are not used, on purpose).
Checks whether Calibre database directory contains some unregistered files and report them if found.
Report duplicates, adding information of which of them are surely safe to merge (because duplicated books are identical or because formats do not overlap) and which require examination.