Finding it would be the frosting on his career, one of the greatest coups in the history of bookdealing. All he knows is that a French collector had it when he escaped from the French Revolution. He tracks this long-dead collector and finds that only his granddaughter yet lives, and is set to inherit a vault full of presumed treasures. The granddaughter, natch, is Jessica. He doesn't want to show all his cards, so he doesn't let her know he's got an ulterior motive for agreeing to serve as her advisor on the treasure vault's contents.
Conflict is the fuel that powers the story.
Well, the B-plot provides the events that keep bringing them together long enough to fall in love and overcome the obstacles to marriage. Each of these events forces them to work together, thereby increasing their trust and respect and desire for each other. John needs to overcome his sense of inferiority that comes from his lowly birth, and he can do this by proving himself the world's greatest bookdealer. Jessica needs to find a openminded man and prove herself his equal, because she can't happily love a man who thinks of her as a silly woman. Both of those are facilitated by the B-plot.