Find an altar and place three ability books onto it. If they are the same type and level then the books will be consumed and the same book of one level higher will be left in their place. If the books are different types or levels then a random book of a completely different type will appear instead. The level of the book will be the lowest level of the three books placed. This gives a use to some of the lower level books that are lying around. There was also an issue fixed with arrow towers not decaying in the underworld. Now go try and create a complete library!