I've been meaning to play this mission for a while now, but the Forced Ghosting and the Hardest genre have discouraged me from doing so as I'm not particularly good at this game. Don't worry, it's not particularily hard unless you play on Expert, the Forced Ghosting does not apply to Normal and Hard. Even if you do enjoy ghosting, playing on Expert seems like a painful choice because you will need to find 8 out of 10 secrets in the first mission and you might randomly fail your ghosting objective for no apparent reason, so consider playing on Hard or Normal.

It's difficult to talk about this mission without any spoilers whatsoever, so just go play it if you haven't already, it took me around two and a half hours total, both missions were roughly equal in length. The following section will contain some light spoilers, but I won't cover the story yet.
This is a fantastic example of how to somewhat reuse a mission without losing any excitement and sense of discovery. The first part has you sneak into a mansion, map it out and find a few secrets, but it's nothing too special. Then the second part also takes place in the same location, but it has undergone some significant changes, more to that in the heavy spoiler section later. You'll recognize all the places from your previous trip, but many pathways are now blocked while others have been opened.

Heavy spoilers ahead!
After you return from your rather uneventful trip to the mansion you get ambushed and locked up in a boiler by the guy whose book you wanted to steal, but you don't spend the rest of your days in there because the book actually contains the Trickster (I think?) and the whole place quickly turns into an even more bizarre version of Constantine's mansion. Something I really liked was seeing all the people from the first part now turned into apes and still somewhat sticking to their daily routine, but now with a pagan twist added to it. You have to go keyhunting, but it's not tedious because the place is so damn crazy that you can't help but be fascinated by the Trickster's work done on the previously ordinary mansion. And even after opening the vault you're still not done, but I don't want to give away everything.