Does anything in this letter remind you of other works by Lewis? If so what?
In case people want to come up with there own ideas first, I'm putting my answer in a spoiler.

How would you describe Screwtape's mood in this letter? Does it change or stay the same?
He seems to be in a dangerous mood and ravenous. That part is pretty much the same throughout. However, he seems to get increasingly angry as he describes what has happened and their lack of understanding about what the Enemy is up to. He seems very desperate at the end as well as discouraged and then delusively hopeful (as well as desperately hopeful). I hope that made sense.
Did anything in this letter surprise you?
That is hard to say since I've read it before. I was a bit surprised by "Pains he still may have to encounter, but they embrace those pains. They would not barter them for any earthly pleasure" because I don't understand what it is getting at.

1. Em, it probably should, but I'm pulling a blank right now.

2. His tone is different in that it's more openly malicious and a bit gleeful (perhaps?) yet also vicious. Screwtape's opinion of Wormwood hasn't really changed, it's just expressed without quite as much subtlety. Like PP said, though, Screwtape is also quite furious---I think he's kind of pleased by Wormwood's downfall, but that it came about this way, he's incredibly displeased about. Also, his temper has worked up an appetite.

3. Not particularly, but I've read it before.
Good point, PP! I'm not really sure what he's referring to either; I'll have to look at that again.

"Pains he still may have to encounter, but they embrace those pains. They would not barter them for any earthly pleasure"

Hmm, I wonder if he meant that though he may still have to encounter his current pain (recalling all of his sins) before God, he embraces those pains, because of the beauty of seeing God help and forgive him, finally being able to see and understand that God was there through it all, showing just how beautiful and forgiving God is.
Just a thought. That is a tuff one...

Does anything in this letter remind you of other works by Lewis? If so what?
''So it was you all the time.'' reminds me of parts in the Last Battle, The Horse and His Boy, and Prince Caspian. It also reminds me of Mere Christianity. For the Last Battle, it reminds me of Emeth meeting Aslan. For the Horse and His Boy it reminds me of Shasta meeting Aslan. For Prince Caspian it reminds of the scene where Peter finally realizes that Aslan was leading them.

How would you describe Screwtape's mood in this letter? Does it change or stay the same?
It's way different, he is not really being harsh but at the same time is being more stern than the rest of his letters. If that makes any sense. I think it has to with the fact that he is using more affectionate terms.

Did anything in this letter surprise you?

I am not sure whether Screwtape is being sarcastic or telling the truth in this chapter. It surprises me that he is using so many affectionate terms in this letter toward Wormwood.

How would you describe Screwtape's mood in this letter? Does it change or stay the same?

Screwtape is upset in this letter. He even grows to a point of despair, but his pride through it all is dominant.

Did anything in this letter surprise you?

That is hard to say since I've read it before. I was a bit surprised by "Pains he still may have to encounter, but they embrace those pains. They would not barter them for any earthly pleasure" because I don't understand what it is getting at.

I think Lewis was just leaving it open for people who believe in Purgatory. Notice specifically the use of the word "may".

Does anything in this letter remind you of other works by Lewis? If so what?

More than anything it reminds me of the happy ending of The Last Battle.

I'm sad that our hero dies in this book, but it is hard to feel too sad, because you know that for the first time he is really alive and has withstood temptation, and is now safe and far beyond the reach of all tempters like Wormwood and Screwtape. And that, almost counter-intuitively, makes this ending a happy one.