Compendiums were very rare and valuable scrolls of various topics written in the Ancient Language. They ranged in topics from geography and philosophy to various spells. It was said that if a magician owned one, he would not part with it. Compendiums often contained words in the ancient language that few or no people knew, therefore making them extremely valuable. Tenga possessed seven compendiums that Eragon observed.
In the chapter "Of Rumors and Writing" in Inheritance, Eragon writes his own small compendium, realizing how useful it would be.