The idea for this casebook is to give the tomes in Arkham Horror a stature more in line with tomes in Lovecraft's literature. In the original stories, tomes were the source of dark spells for summoning awful creatures, and the source of knowledge for scholars opposing them. This casebook adds some mystery and power to tomes, giving a unique character and story for each one, and a set of new spells and skills that can be derived from them. You can get the new expansion from the Arkham Investigations casebook page.
Here are some teasers from the expansion: