Why is the AI offering a one-for-one trade of Great Works of the same type?

Solution 1:

Great Works have a theming bonus.

In order to qualify for the theming bonus, the housing building must have its slots filled with Works of the same subtype by different Artists.

So it may have been a matter of them trading a "duplicate" artist in exchange for one whose work they did not yet have. The same could apply to ensuring the same subtype but in your case it's a book-for-book trade.

For completeness' sake, artifacts have a different ruleset for theming bonus:

Each Artifact will have an Era and a Civilization. In order to qualify for the theming bonus, the containing building must have three artifacts from the same era (ancient era, classic era, etc) but created by different civilizations.