On inversion

Hello! I would like to know the reason of inversion of sentence (a), which I suppose comes from sentence (b). I wonder if it is because the subject noun phrase "an explanation of how the rich-poor gap has come into being" is too long to come first as a subject phrase. In that case, which would be better, (c) or (d)?
a. Missing from the book is an explanation of how the rich-poor gap has come into being.
b. An explanation of how the rich-poor gap has come into being is missing from the book.
c. Missing from the book is an explanation of the process. d. An explanation of the process is missing from the book.

I appreciate your help in advance.
LP