It's not the Great Library, but something fairly similar: Oxford University.
- it's the oldest University in the English-speaking world, yet it is the youngest in the game because you have to build universities in all your non-puppets to get it.
- these other universities are the metaphorical "siblings", and "learning from them" is a clue that we're talking about something related to education.
- the "something expensive" is a free tech, which often means the most expensive one available. This bonus is a one-time benefit, hence "just this once".
- the "fast learning" is, again, the free tech, and the foreign book collection references Oxford's theming bonus requiring the Great Works of writing to be of foreign origin.
I swear I thought this one would've been easier!