You might wonder what the heck do Cecily’s and Gwendolen’s diaries have in common with Miss Prism’s three-volume novel – other than the writing part. Well, the writing part is actually important. Think about what you do when you write. It’s always a very personal activity, because the way you string the words together is completely your creation. It’s your thoughts that are put down onto paper. Your writing is an expression of yourself. So it’s no surprise that some people want to keep their personal thoughts private. Hence, you have a diary. Many people’s thoughts and desires are irrational; instead they’re very idealistic.
This is the point in The Importance of Being Earnest. Almost any type of book or writing, with the sole exception of Jack’s Army Lists, reveals someone’s wishes or dreams. Cecily’s diary meticulously documents her desire for a lover and future husband named Ernest. It even includes imaginary love letters. Gwendolen’s diary does the same, minus the letters. Lady Bracknell’s notebook keeps tabs on men who have the potential to become worthy suitors for Gwendolen’s hand. Most of the content in these pieces of writing is unrealistic at best or fantastic (in the fairy-tale sense) at worst. But these thoughts are kept private.
Miss Prism’s three-volume novel, on the other hand, reveals what happens when one tries to impose an impossibly idealized world onto gritty reality. Miss Prism probably wrote her novel in her younger days, when she was dazzled by other romantic and sentimental stories published in the same "triple decker" genre. Thus, her writing could have been a sort of diary, a projection of a perfect inner world – her deepest desire – put into words. But everything fell apart when she tried to publish it – pushing it into the public sphere. It caused her to forget her real responsibility – baby Ernest – while she was daydreaming about future success. She lost her job over it and was pursued by Scotland Yard. Her actions made her a criminal. And Lady Bracknell returns years later to haunt her about it.
So the diaries and three-volume novel of our female characters represent the innermost fantasies of idealistic young girls, dreams that clash directly with reality. Miss Prism puts it best with her quote: "The good end[s] happily, and the bad unhappily. That is what fiction means" (II.15). You might want to counter, that very few things actually end happily-ever-after in the real world.