Once upon a time, all the legions featured psyker-warriors within their ranks. These librarians would be disbanded under the Edict that followed Nikaea (otherwise known as the Trial of Magnus) in the main part, but came back with interest paid during the Heresy.
For reference purposes, the old Inferno black book listed those legions in favour and against psyker battle brothers. Those legions against the use of librarians are:
Candidly, even among these legions there are librarians. The Space Wolves used rune priests to channel the power of Fenris (no doubt causing Magnus to giggle at the claim that they were opposed to psykers) and the last cadre of World Eaters librarians certainly persisted into the Heresy despite Angron finding them more than headache inducing to be around.
Mortarion hated psykers due to his personal upbringing on Barbarus. Yet psykers persisted even within the Death Guard, and remained hidden in some famous cases. I don't have much to say about the Iron Warriors and Iron Hands avoidance of psykers though.
The most obvious strength for Librarians is access to psychic disciplines. But there's one item of equipment that is easily overlooked here and that is the psychic hood. Take it. Every time. Yes, it chews up points that you might have used elsewhere, but for that army that you will face every so often that uses psykers, you will be glad that you did.
Not many realistically other than the standard issue of instant death.
Warpstone Flux Rating:
4/5. The rules are strong. They also allow access to psychic disciplines for all legions, and the use of valuable psychic hoods. For some legions, this is the only psyker available.
Difference to First Edition.
Neutral. The rules are comparable. That said, the whole approach to psykers and psychic powers has radically changed -- and changed for the better -- and it is worth stating that. I will look at psychic powers distinctly later on.
Legion Librarian with Force Staff, Psychic Hood (120 points).
My baseline offering. Take with a squad in a transport.
Legion Librarian with Force Axe, Psychic Hood, Jump Pack, Plasma Pistol (150 points).
More of a lone star threat, but also good to rock and roll with an assault sqaud.
Legion Librarian in Cataphractii Terminator Armour with Force Axe, Psychic Hood (145 points).
Take a combi-weapon to taste, but otherwise ready to go with a terminator squad.
Tartaros terminator armour will drop the points cost by 10 here, if needed to fit in with such a squad.