1. A method of presenting eBook content on an eBook reader device, the method comprising: displaying a portion of the eBook content on a display screen of the eBook reader device under a set of display conditions including a font size and a line spacing; and providing on the display screen of the eBook reader device one or more invariant location reference identifiers corresponding to the portion of the eBook content displayed on the display screen, the invariant location reference identifiers being separate from the eBook content, such that the eBook content is unaltered by the provision of the location reference identifiers, wherein each invariant location reference identifier is provided along with the corresponding portion of the eBook content, regardless of the display conditions under which the portion of the eBook content is displayed.