The Boke of Palimpsest is a fictional book introduced in House of Many Ways. It is currently owned by Wizard Norland.
Palimpsests are pieces of writing material (such as parchment) whose original content was erased, so that new content can be written to it. They were common in the Middle Age, due to the elevated price of parchment.
The contents of the book seem to change from time to time, since Peter could not find one of the pages Charmain had previously found. This behaviour aligns well with the concept of palimsest - If the book was made with one, each page could contain more information than what would be shown at a time.
According to the Witch of Montalbino, the book would give a magic user the hability to control the four elements, although fire would only be granted to the trustworthy. For the book to work, the reader had to be somewhat apt to use magic.