Bruji

I've downloaded ~100 books, only 14 of which have Dewey decimal numbers. Can I do something different to complete this field?

The information depends on the databases. The Library of Congress is good about also carrying the dewey information. You can run get additional information by using "Book -> Update from -> Library of Congress". Since most books will have an ISBN it will use that information to do the search and get an exact match and add any missing details. The same can be done from the gear button on the bottom right of the add/edit window if you want to add more info after finding a result.