my wife sharing my iTunes content with her different Apple ID

See Authorization vs. association - Apple Community for the difficulties that can arise when you try to access content from different Apple IDs on the same computer. It is possible that you could Set up Family Sharing - Apple Support to allow you to share each other's purchases, but I'm not certain that audiobooks are included. If she has her own computer you can physically copy the files from one computer to the other, authorize it for protected content if required, and then copy to the device.

tt2

In a roundabout way it comes to the same thing. Apple have things set up such that media on an iPhone can only be managed from a single computer/library, even when manually managing the content. Attempting to add content from a different computer will typically raise an erase and sync warning unless you are using a clone of the library previous used to add media. This is likely why any drag & drop attempt failed. Switching IDs on the device in order to access past purchases from a different account gets you involved in the 90 day association lock.

tt2