I, Marcellus is a book written by Marcellus Pye. It is divided into three sections in respective order, 'Alchemie', 'Physik' and the 'Almanac' and covered in soft leather and gold leaf, located on the Sealed Shelf in the Pyramid Library.
Septimus Heap didn't understand it but Marcellus claimed that it contains simple steps to transmuting gold and the key to eternal life.
The section contains complicated formulas for medicines, linctuses, pills and potions. There are also explanations of the origins of many diseases and detailed drawings of the anatomy of the human body.
The section contains a day-to-day guide for one thousand and one years after the book was written. The Almanac is where Septimus hid his note to Marcia Overstrand, in the entry of the day that he disappeared.