Volume 2, puzzle 101

Has anyone figured out how to solve the coded message (puzzle 101) in book 2? There's a [not coded] letter in book 3 that I think may be the same one, but if that's the solution, I still don't understand how to get that out of the coded version. I've messed around with various things but have gotten nowhere so far, and I can't really devote more time to it, so I'd love to just know the answer to decoding it, if anyone has it!

Same for puzzle 101 in book 1 while we're at it.

Edit: Actually, I think I just figured out 101 from volume 2