Orígenes del español: estado lingüístico de la península ibérica hasta el siglo XI

Classic work by Ramon Mendendez Pidal on the development of Spanish, with plenty of comparisons to developments in neighbouring Astur-Leonese, Navarro-Aragonese and Mozarabic.
The PDF has large six colour maps provided in a much higher image quality (resolution) than the rest of the pages, even if the default view is similar (this means the maps can be zoomed in with good results).