Our design for the Aldiko Book Reader is to store books in external storage. The application will tell the device to save books in external storage. However, it will depend on how the device defining external storage. We have noticed that some devices set aside part of the internal memory as external storage purpose. In this case, even the Aldiko Book Reader asks the device to save books in external storage, books will still be saved in the part of internal memory that serves as external storage purpose.
Currently, there's no way to change the storing location of books manually.