I felt gratitude but did not know how to say so.
-- Jack Agnew ("Letters")

Importance: In Jack's first letter to Louisa, he says that his reason for contacting her is simply to express gratitude for her work in the library. However, this motivation appears rather flimsy, and it quickly becomes apparent that Jack likely has some romantic fascination with Louisa.

The idea I won't ever see Carstairs again makes me thinks I can say anything I want...So I will say I love you.
-- Jack Agnew ("Letters")

Importance: This is a pivotal moment in Jack and Louisa's correspondence, as Jack openly declares his love for Louisa. However, it also foreshadows the later revelation of Jack's engagement to Grace Horne, as Jack is motivated in his declaration of love for Louisa by the belief that he will likely die in combat.

Oh, it was not a matter of principle. That I kept the Library open. It was a more...
-- Louisa ("Spanish Flu")