The book is not a Sacco-Vanzetti encyclopedia. I touch only in passing on some important subjects that I considered secondary to the book’s main goals. I know that the book could, as Barbara Tuchman said of The Proud Tower, “be written all over again under the same title with entirely other subject matter.”
(Source: Susan Tejada: In Search of Sacco and Vanzetti, p. x.)
I am not sure what is the meaning of the phrase "secondary to" in my clause. In the case that it has the standard meaning – not so importatnt, not so relevant – the sentence niggles me a little. The book is supposed not to be encyclopedia and the author focuses only on the lives of Sacco and Vanzetti and importatnt subjects are mentioned only briefly. But the reason for their being mentioned should stem from their importance to the book's main goals so the usage of "secondary" seems to be odd to me.