Re: Translating hardware to software, plug compatible is *not* the equivalent of a API.
No, API's don't fall under copyright by virtue of being published in books. Here's the relevant section of the law ( https://www.copyright.gov/title17/92chap1.html#102 ):
102. Subject matter of copyright: In general (28)
(a) Copyright protection subsists, in accordance with this title, in original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device. Works of authorship include the following categories:
(1) literary works;
(2) musical works, including any accompanying words;
(3) dramatic works, including any accompanying music;
(4) pantomimes and choreographic works;
(5) pictorial, graphic, and sculptural works;
(6) motion pictures and other audiovisual works;
(7) sound recordings; and
(8) architectural works.
(b) In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work. [emphasis mine].
An API is a "...procedure, process, system, method of operation..." and so categorically excluded from copyright protection regardless of how it was recorded, in a book or in anything else for that matter.
Fair-Use, compilations, none of that applies as a matter of law. It's just that this particular appellate court (with a history of rewriting law and getting smacked down by the supreme court) overrode the correctly descided lower court, twice, to rule otherwise.