So, about those top-secret files Capt. Montgomery mailed out before he was killed. There are only so many people he could have sent them to, right? Monty, for all his failings, wouldn't waste a stamp sending Johanna Beckett info to someone who couldn't do something useful with it.
One Castle fan thinks he's solved the mystery and wrote this article detailing his findings. We enjoy really astute/obsessive Castle observations, and this guy doesn't disappoint. He basically slowed down the scene of Montgomery addressing the envelope and watched it 10 billion times in a row. Result: The strokes of the late Captain's pen seem to indicate Richard Castle is the lucky recipient!
But we kinda all figured that, right? Castle basically re-opened the Johanna Beckett case, and he's, uh, pretty good at putting clues together. The bigger question is who else could it be? Probably not Lanie or Martha, because … we just don't think that's who got them. Maybe Ryan and Esposito, but since they live in separate places, Monty would have had to pick one, and he didn't seem emotionally stable enough to make a decision like that. Beckett is the second most obvious choice after Castle, but we all know how concerned Monty was for her safety (sort of). Sending that heavy stuff to he directly might not end well.
So, the best guesses are, in order, Castle, Alexis or the mayor. (But it's probably just Castle.)