Mantano has a sqlite database where it keeps all info from the books. So, deleting this database should be enough.
What you can also do, is select all books, select "delete", but don't delete the physical files (it'll ask about that). After that, you can reimport them again. (that's what I did to clear up my collection-mess...) All can be done by a "select all", so no need for one at a time...
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