Sic Transit Bibliotheca Mundi

All things are possible.
All things are known.
Nothing is remembered.

In the Biblioteca Mundi setting there are spells available, or at least working theories, on how to do anything.

However that was during the height of the Great Library three millenia ago.  Since that time all the books have been checked out and none returned.  Or more accurately:  The library has been sacked hundreds of times and it's pieces are scattered among guilds, wealthy collectors, misanthropic magicians, and the tombs of dead kings and sorcerers.

If you want to cast a spell you can hope the information is easily available locally (unlikely), or spend fifty years researching it yourself (Yay! A life well spent.), or go raid the dungeon where rumor has it that a treatise on similar effects was once stored.