The books should be there until they are bought, AFAIK.
You should be able to use the Save Game Editor
to replace the books. It is a Windows program, but apparently will run under Wine in OSX. I'm not sure if they are listed in the editor, but if not you could make a copy of an old save, edit it to give yourself gold, load the edited save, buy the books, save and then use the editor to export the books in that new save and import them to your latest save.
Please email email@example.com, with a brief description and the report.zip file generated by the D:OS Analysis Tool
(for the PC version). The report will contain system and game information, as well as your saved games. To reduce the file size, you can delete most of the saves from the zip; keep your latest save, the latest save at level 5 when she had the books and the oldest save on level 6 when they were missing. The 2 closest saves with and without the books should help narrow down what happened to them.
For Mac, to get system specs you can use the 'System Report' button from the 'About This Mac' Apple menu item or the 'System Information' app (in the utilities folder).
Saves can be zipped by right clicking the folder(s) and selecting Compress, in the folder:
~/Documents/Larian Studios/Divinity Original Sin/PlayerProfiles/<ProfileName>/Savegames