I wrote a quick how to on troubleshooting files to discover their issues
https://forums.adobe.com/message/5466671#5466671
In your case you have “divided and conquered” and found out that you can export half the book and then the other half and it works.
I suggest continuing this practice by exporting 2/3 of the book and see if it crashes – and if it does then try narrow down by exporting back 1 chapter at a time to where the problem chapter may be.
Let me know how you get on.
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By the way – it might be a better workflow to export all your book files to IDML from your older version of InDesign
– then open those IDML files in the later version of InDesign
– then generate a new Book File and add the new files to this
https://www.kahrel.plus.com/indesign/batch_convert.html
This script will allow you to make IDML files from a Folder of InDesign files make sure you’re in the older version of InDesign (play with it and you’ll what I mean)
Speed things up for you.
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If you’ve already made a load of changes to your book and this is too much work to go back
I suggest you export all your Files from your latest version of InDesign with all your changes to IDML.
Open them in the latest version of InDesign – resave them as InDesign Files.
Then Create a New Book in InDesign and try the export.
Honestly it won’t take long.