Why not "léann iad na leabhair" also ?
Siad is used when "they" is the subject of the sentence, iad is used when it is the object (usually "them" rather than "they").
I accidently preesed it huh so annoying
Is léid archaic?
It's a form that is really only used in Munster Irish. I'd hesitate to call it archaic (somebody will get their nose out of joint) but it's not always used even in Munster these days, and is unfamiliar to many learners of the language.