That this will be a long dissertation on Darcy's all important letter, the letter that is a chapter, the letter that explains everything, the letter that, with the proposal & Lizzy's subsequent considerations on the letter, is the turning point in the story.

I won't do a line-by-line analysis of the letter--even were that in my power, it would make this too long, & besides, the next chapter gives Lizzie's line-by-line analysis. I will briefly summarize the letter & then make some observations that I think might cause some interesting discussion.

Summary: Miss Bennet, don't think I'm going to give you another opportunity to shoot me down but I have to explain some things.
First, yes I did my best to separate my buddy Bingley from your sister. Your extended family are nobodies, your parents & younger sisters are fools & besides I never got the impression that she was that into him.
Second, yes, my Dad liked Wickham & wanted to give him a good start in life as a clergyman. My observations of Wickham let me know that having him in charge of a parish would be like letting the fox guard the hen house. He asked to be paid off & I did so, but when he'd gone through that money he came back asking for the living & was refused, & angry, then he tried to run off with my sister to spite me & get her 30K pounds, but she spilled the beans just before.
So that's that.
God bless you.

My observations are:
Why a letter? Darcy gives one answer to this in the letter "You may possibly wonder why all this was not told you last night; but I was not then master enough of myself to know what could or ought to be revealed." The letter format certainly allows Darcy to describe all the events and to tell the story from his perspective in a coherent way, without the interposition of questions or arguments, which might make his exposition longer or more confusing if JA had presented it as a conversation, or as the collection of pieces of uncontrovertible information derived from various other sources. An explanatory conversation could have turned as ugly as the proposal did. The letter keeps it civil. It also keeps it between Darcy & Lizzie. Col.Fitzwilliam is standing by to verify what Darcy says, but the communication is between Darcy & Lizzie. I think this letter also creates tension between L&D that might not have otherwise been possible from a conversation or observations of activities. The letter is about the suspense & excitement of the written word, about the initial confrontation with what's on the page followed by continued interaction with it, for both the receiver and the writer. In writing this letter, Darcy makes himself vulnerable in many ways. He acknowledges that he did propose & that he was refused, he admits the need to explain himself & even that he may have misjudged Jane's affection. He also has to lay open some family secrets & speak of events that cause him shame & sadness. Moreover, just writing the letter is an admission of his need to be at least understood, if not loved by Elizabeth. In his intro to the letter he gives Elizabeth a huge compliment. "You must pardon the freedom with which I demand your attention; your feelings, I know, will bestow it unwillingly, but I demand it of your justice." He is honoring Lizzie with the (traditionally considered) masculine value of being able to subjugate feelings to justice.
I wonder if JA was making a comment on the importance of the written word in making a letter and its revelations a major crisis in her novel.