There are quotes from The Book of Shhh at the beginning of some of the chapters in Delirium, and Lena mentions it as though it's a sort of religious tome, saying, "defacing or destroying the Book of Shhh is sacrilege" (14.80).
We can only piece together what it is through hints dropped in the text. Lena says it "tells stories of those who died because of love lost or never found, which is what terrifies me the most" (1.11). It seems to serve as both a guide to life, and a way to instill fear in the general populace.
We're not sure where it got the silly title, though. This is Delirium, not Hush, Hush.