I'd recommend stating what your "old" references version is in order for people to guess what you know.
Libraries don't do much in practical terms, although it's said book contents can shift values (someone wanted to promote a clothes less fortress by using books about the value of shamelessness). Research is currently just flavor. Reading books satisfies the need for introspection, though. As I haven't used Masterworks I can't comment on any similarities.
To write books you need blank quires and one or more scholars appointed to the library (visitors can also write books). Scribes copy books, but don't write originals. I'd recommend checking what's on the wiki, though.
Tavern keepers and performers are indistinguishable in practice. They push booze down peoples' throats which increases the frequency of lethal "non lethal" bar room brawls without justice involvement, as well as death by vomit suffocation. They also get visitors, including vampires and goblins, to drink (with goblins having a higher alcohol poisoning death rate than dwarves). If you want to keep as many alive as possible, but still have a tavern keeper, you should probably go for slow, lazy, deeply religious (spending a lot of time in temples), unhelpful, ... If you want to kill as many as possible you should probably go for the opposite. Tavern keepers and performers perform about as much as any idle dorf.
Visitors are new, and can petition for residency. Mercs will never go further and can only be used in military squads led by a citizen. Other petitioners can petition again for full citizenship (I'm not sure they'll ever stay as residents without petitioning without a bug preventing them). When they become citizens they're completely equal to your dorfs in terms of control, etc. Thus, a naturalized performer can lead a merc squad. It's not uncommon for visitors to be cursed (I've never seen a lycanthrope one, though, but others have). If you manage to get petitions from couples you can get children who're also full citizens, for a truly multi racial fortress. Performance troupes are a pain, since there's no info on which visitors a troupe contain, and troupes usually contain members not present. When a troupe petitions you get them as a bag of unknown individuals, give only the troupe name. I don't think anyone knows what happens if a troupe member happen to arrive later, though.