The book was found in the library two terms ago, where Marcella was entranced by the green stones on the cover, once she went on a rage through the library, she sat there and looked at it blah blah blah... and opened it and used her parseltongue to be able to read it. We then pondered over what the "writing was" and came to the conclusion it was a potion for the dead (though of course Emma knew this *pokes*) and then marcella left. Thats basically how she got held of the book. Does that help? Maybe? Yes? No?