Your best bet is WD139/140 for information relating to the
Adeptus Astra Telepathica and, for example, novels such as Farrer's
Blind for more "flavoursome" information on psykers themselves. Those are the most obvious sources, along with the
Inquisitor rules (including the Specialist Games articles on psykers). Of course, the singular best source of information is discussion amongst the fans themselves. We're kind of the open source to GW's "Microsoft".
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Kage