I fail to see how it's not copyrightable as such. It appears to be a collection of facts (the interface of the implementation). These are not copyrightable, unless it meets the following:
"the work must be a collection and assembly of pre-existing material, facts, or data; the work must contain the selection, coordination, or arrangement of those materials; and the work is created by virtue of the particular selection, coordination, or arrangement of an original work of authorship."
I can't see how "arrangement of those materials" and "virtue of the particular selection or arrangement" isn't obvious here?
Oracle is pursuing the path where this case is literally about the contents of text files with words, which are supposedly well defined and understood under copyright law.
That is a bit disingenuous. Using your analogy it would be the subject sentence of every paragraph in the book.