Your FrameMaker documents may have many variables that are common across all files in a book. If you’re using unstructured FrameMaker, you can set these variables in one of the files in the book and then use the File > Import > Formats command to import these variables into the other files.
If you also plan to import paragraph format properties from this file, ensure that it uses all the common styles used across the book. For example, if some of the files in the book have <Heading 4>topics and the file you’re importing formats from has none, you may notice formatting (and numbering) anomalies in the final PDF. In such a case, it is recommended that you set and import variables and paragraph formats from a file that has <Heading 4> topics.
See the “Import formatting properties” topic in FrameMaker Help for more insight.
If you’re using structured FrameMaker, you can have book-level attributes and use them (for instance) in running header/footer variables using the <$attribute[AttributeName]> or<$attribute[AttributeName:ElementName]> building block.