There are only a handful or so of official Things in Eric, all of which are books as you can see in this chronological list. See them in alphabetical order here.
The Joy of Tantric Sex with Illustrations for the Advanced Student – By A Lady. Perused by the Librarian.
Some Little Known Aspects of Kuian Rain-Making Rituals – Unfinished(?), seven-volume treatise on the subject by Archchancellor Ezrolith Churn.
Names of the Ants – By Humptemper. Once used by the wizards of Unseen University in an unsuccessful to attempt to stop a haunting.
Demonologie – Studied by Eric. Contains a Riinjswin and a Winswin but not a Rincewind. Those found in the tome had more wings than the wizard.
Mallificarum Sumpta Diabolicite Occularis Singularum – The Book of Ultimate Control. Eric owns a copy. The wizards do too but never read it.
Octavo, The – The most magical of all books, located in the library of UU. It is “the last remaining copy of the Creator’s own grimoire.”