The Profession (Librarian) skill is slightly different than your standard skill only in that as a skill it
For those who are unfamiliar with 'skill tricks', they appear in The Complete Scoundrel book and their purpose is to allow a character to do a little more spectacular things because they have a certain level of ability in a skill. For example, if your character has a prerequisite of 5 ranks in the Heal skill, you can spend 2 skill points upon leveling up to gain the "Healing Hands"skill trick which allows a character to heal someone 1d6 when they do a heal check to stabilize someone.
Initially, I thought the Profession (Librarian) skill could be one that replaces Knowledge (Anything) checks, but that would just let a player stack points in the profession and be a wealth of knowledge. Next, I considered giving the player the Bardic Knowledge skill using the Profession (Librarian) skill in lieu of the bard level, but then that would make a player nearly-omniscient by level 10 (as even the hardest Bardic Knowledge check is DC 30). So my compromise was that the Profession (Librarian) would on it's own do nothing but feed into a skill trick.
will not be horribly useful. However, taking levels in it proves to be quite beneficial if you are a player that likes taking skill tricks in lieu of your standard skill points.
The skill trick, which I call "Wealth of Knowledge" [Mental] would allow a player with 9 levels of Profession (Librarian) to use the Bardic Knowledge ability using their own class level as a Bard would use his/hers.
I believe this is a good balance because it means a character cannot take this skill trick until at least level 6 which means a player can still choose to be a Bard and gain that ability earlier. The rationale behind this skill is that a librarian that has been doing that job for a while will have picked up some obscure knowledge here and there so they could help provide similar information that a Bard would have picked up by traveling around and telling stories.