An imported (or opened and converted with LR5) catalog from LR4 into a LR5 catalog containing a book created with LR4 won't export to PDF or Jpg.
If you start a second book with the same images in the catalog this will export.
I didn't actually reproduce this issue.
What I saw is if I wait for a long time after I press 'Export Book to PDF/JPEG' button, the dialog was finally shown out and the book was exported successfully.
I would like to get more info about this issue:
1. How many photos do you use in the book?
2. What do you mean when you said books won't export? Do you mean the export dialog won't be shown out?
3. What will happen if you wait for a very long time after open the Lr 4 catalog? Because when you open / import the Lr 4 catalog in Lr 5, Lr 5 needs some time to generate the preview data for each photo. It takes long time and no obvious indicator or process bar in the UI.