David, I don’t think this kind of capitalization is grammar (although the specific rules of title casing do reflect grammar, i.e., lowercase minor words).
Any phrase in a book may be used by the author as a chapter head or subheading. It may comprise a grammatical sentence, a fragment, or a single word. But, conventionally, it would be written in title case.
Or, not. Because title casing reflects the superficial form of some copy, not an inherent (e.g., grammatical) aspect of it, a designer may choose to give it distinctiveness or visual emphasis by other means. Especially on book covers, title pages, and in wordmarks on movie posters, we might see titles in all caps, in sentence case, or all lowercased.