This is not typically my pairing, but you've definitely sold it to me here. They're both intelligent and passionate in their own ways, and each able to respect their similarities and differences.
I adore Bogdan; you've really brought this OC to life -- and made him highly entertaining. His interaction and gruff comradery with Charlie is so fun to read.
I think, though, that Snape and his Attitude (capital A, 'cause it's an entity unto itself, LOL) steal the show, even when they're only present in the form of letters. Hee. His distinct personality comes though so clearly in his letters. And his hints are just so obvious -- to HIM! LOL His annoyance that Charlie didn't get it -- awesome. XD
Snape's perfect in his subtlety. He shows his increasing caring and respect for Charlie in small ways. But when it's important, the full force of his passion is there. His defensiveness when Charlie is leaving and initially suggesting that they could continue their affair is so like him, and then...
"That is true." Snape pressed a kiss to the sensitive skin beneath Charlie's ear. "Nonetheless, I find myself in a conundrum. I had expected my first to be my only. But now I have you, and you simply refuse to act like anyone else."
...gah, awesome.
Love that Snape challenges Charlie throughout this story -- both in present and past -- and that Charlie's clearly intrigued by it and attracted to it. And Charlie can totally keep up, meet every challenge, and present a few of his own. They're well matched and complement each other perfectly in this. Snape's little hints of doubt and insecurity when he finally brings Charlie to him... and that Charlie (finally) picks up on them and gets it... and that so much just doesn't need to be said between them... yes, yes, yes.
Snape's letter to Macek -- bwahahaha!
The sex... guuuuuuuuuuuuuuuuuuh. Just, yes, monitor-meltingly hot and intense. Bottom!Snape, yay!