The book is located in a specially sealed room in the basement of the Unseen University's library chained to a lectern, for safety of the browsers, not the book. Despite its importance,
The book itself is not too impressive, its appearance was rather plain with only a picture of Bel-Shamharoth on it's cover. The spells are what make the Octavo a dangerous thing.
The spells have a mind of their own and it is not the reader who might read the book, but it is the book which is reading the reader. Rincewind somehow caught the interest of one of the eight spells, which then decided to live in Rincewind's head for some time.
The lectern in turn is in the centre of the extremely potent Seal of Eightfold Stasis as a further safeguard against the Octavo escaping. The chain is locked with eight padlocks, one lock for every head of the eight orders of wizardry. It is very unsafe to stay in the room for too long and it is not recommended at all to read the book.
The spell that was lodged in Rincewind's head has eight words. Seven of them are: Ashonai. Ebiris. Urshoring. Kvanti. Pythan. N'gurad. Feringomalee. -. In The Light Fantastic, when the time comes for him to say the Spell for real and save the Disc, he botches it, and Twoflower and Bethan have to correct his pronunciation for it to take effect. (Rather than appearing octarine, the mispronounced words appeared to be a dingy off-colour brown).
When the eighth spell was falling irrevocably off the disc inside Rincewind's head, the seven remaining spells worked together and cast a Change spell that changed the entire disc and ensured Rincewind fell to earth again, landing up a tree in Skund. Part of the fall-out from this spell affected the Library immediately above. It was therefore directly responsible for changing the Librarian into a small sad orang-utan, sitting forlornly in a puddle of pineapple custard.