Dark Day
From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
Dark Day is a novel written by Edward Prima.
Given the context of the discussion between William Adama and Laura Roslin, notably her reference to A Murder on Picon, it is likely it may be a mystery novel.
Roslin considers it to be "one of those classics I've never gotten around to reading". Adama, who had the book in his personal literary collection on battlestar Galactica, gave it to her as a gift.