If the Librarian in our party becomes even more powerful, I will have to move the Emperor himself against him.

Got already enough problems with him, last week he came up with this Power Dome thingy... jeez...

Librarians though pay a heavy price for their power. Upgrading their psy rating is horribly expensive. Most classes dont get upgrades which are above 1200Xp, Librarians though, Psy Rating 8 for example costs 1500, 9 for 2000 and the final tier is a massive 2500xp. As well, you really cannot afford to make a librarian super high psy rating and good with their intellectual capacity or forbidden lore, which they can also do well on. You have to specialise with them.

Also in consolation, powerful as he might be, just throw some Psych out grenades or Witch Bolt rounds against them (-1PR for 1D10 rounds per hit) that and any librarian could at any time risk obliterating themselves with Perils of the Warp.

Librarians though dont need much in the way of gear, they have their mind, and that is more potent than any bolter.