The Book of Peridot was under the skulls. Being slightly larger and thinner with less pages than the Book of Rose Quartz, it felt rather light in Bastian's hands, even though the thing still radiated with ancient power, in a more immediate way than the pirate captain had expected.
The peridot on the cover was a rather large one, making the monetary value good even without considering the immense value of the unique spells inside.
Some spells could not be copied. Bastian did not actually knew if they were cursed, but in any case it was supposedly literally impossible to trace the same patterns, the same words, onto a new page. This was part of the reason why such books were valued in occult circles, as this ancient heritage would have been lost if not for Bastian's incredible stupid bravery and his neck muscles.