"The conclusion to which we have come is, that blank account-books are not the subject of copyright; and that the mere copyright of Selden's book did not confer upon him the exclusive right to make and use account-books, ruled and arranged as designated by him and described and illustrated in said book." [1]
I believe based on this reasoning, copying APIs or copying code is a distinction without a difference, so long as it's in the context of calling library functions.
The Baker opinion can of course be used as argument pro-Google. But it is also just an "opinion". Each case must be decided on its own.
I believe there is a big difference between scale and structural complexity with the (inter-connected) Java API and some text-book examples of accounting layouts. At some point quantity becomes quality.
In both cases, they would have "copied" the API and both seem to me to be exactly the same case as in Baker/Selden.
So that case was about illustrating and explaining a method of doing accounting. There was no textual matter copied.
In Oracle vs. Google I believe the source-code of the APIs was copied.
Even if one would consider a way to do accounting an "API" an explanation and illustration of an API is not the same as the "API".
Explaining or illustrating a "structure" is not the same as copying that structure. Note that copyright applies to actual "works of art", not to abstract "methods". An API (as referred to in this case) is not an explanation of a way of doing things but an actual constructed artifact, source-code.
Think about books. Their content is copyrighted. You can not copy their content. But nothing prevents you from explaining and "illustrating" the content of a book, as long as you don't copy significant portions of the structure and ordering of its verbatim words.
True. Neither are 'ways of calling library functions" copyrightable.
But in the case of Java API what was copied was the actual Java-code in which the APIs are expressed.
Because the APIs are "fed" to the compiler as Java source-code, it is clear what was copied was not just the "way library-functions are called".
What was copied was Java SOURCE-CODE which allows a Java compiler to ENFORCE and INTERPRET such calling conventions.
Even if the API was rewritten in another programming language it would be a copyright violation if large parts of the structure of the API meaning how the methods are organized into classes and interfaces and how they refer to each other, was copied. That would clearly be a work of art that deserves copyright protection, in my view.