And to whoever is googling "master letters emily dickinson sparknotes", YOU DON'T DESERVE TO EVEN KNOW WHAT THAT IS! Your teacher/professor goes out of their way to give you this incredible assignment, and you need Sparknotes? THERE ARE THREE OF THEM. And they will change your life.
Read them, and read everything you can find that was ever written about them. Trust me.
And if you're just reading them and confused about the meaning, then kudos to you for trying to sort it out, but -- trust me -- there are much better sources. Although not nearly enough of them, I know (this blog being the fourth result on Google for the text is evidence enough of that). But here's a good place to start.
The paper I've written is little more than undergrad drivel (albeit very passionate drivel -- my entire thesis, which I was finishing at the time, centered around letter writing and poetry). The professor presiding over the I.S. I wrote it for wanted me to put it up for publication, but I didn't really see the point in that. Anyway, given how little there was available to me when I was researching the letters, I thought throwing the paper up on the internet couldn't hurt, if other people were looking.
Better yet, embrace the fact that no one's given the proper amount of shit either about Dickinson or the letters to pulverize the meaning of them with academia yet, and make of it whatever it is you feel you should.