How can I kill a librarian without attracting more of them?

There are 3 main ways of dealing with the Librarians.

First, according to the Metro wiki article on Librarians:

All librarians in Metro 2033 and Last Light have been tested, and can all be stared down with enough practice, skill, and luck. The player can have Artyom or the Stalker avoid all by following Miller's advice and staring them down; stare into one's eyes long enough and it will back down and retreat for a short period.

So it seems that combat can be avoided altogether by staring them down.

However, if combat is necessary, then the same wiki page suggests using hit-and-run tactics and getting out of range:

If awoken or otherwise engaged, the next best tactic is to hit and run, sidestepping around a pillar or jumping onto something elevated where they cannot reach.

If fighting becomes necessary, the wiki suggests using point-blank shotgun shots to the head, as well as MGR-loaded or pneumatic weapons, due to the Librarians' high health.

Finally, one can try to flee from the Librarians by abusing the corners and tight-spaces in the Archives:

...it's often easy to outrun librarians since they cannot turn corners as well as Artyom due to their size... The librarians also have trouble following Artyom if he runs into confined spaces like stairs or catwalks...

So if running from the Librarians, use and abuse the tight spaces of the Archives like the stairs or the catwalks. These spaces should not only help you flee, but also give you good, relatively safe locations from which you can attack the Librarians.

All of these (and especially killing the Librarians) will be harder on hardcore mode, but still possible.

As to avoiding attracting others, the only thing I've been able to find comes from the Archives wiki page:

Equiping the stealth suit and a silenced weapon can help stealthy players to avoid fighting the librarians. The player should watch for object that can attract librarians even they have such equipment.

So it seems that avoiding attracting the other Librarians is going to be pretty tough, especially if you try fighting them. I would suggest staring down the Librarians - though it can be difficult, I found it to be the safest option.