HOLA!
I love that we've got linguistic conversation! that's definitely my area of interest. This year I took a course about the history of the language, and I have some great references for all you aspiring linguists out there, or for anyone who's curious.
My fave is Ralph Penny, I've used 2 of his books in 2 different classes, and they are VERY useful. Both have great appendices if you're searching for the evolution of specific sounds or words, etc. Here's the one about history of the language (and there is a version española as well)
'A History of the Spanish Language' by Ralph Penny, 1991
there's also a great one by Paul Lloyd, but I found it a little harder to follow than Penny.
'From Latin to Spanish', 1987. it's also a great reference, and has a version española.
then, the mother of all : COROMINAS. This is an etymological diccionary, and has the history and evolution of a bizillion words. I used this for several papers, it's amazing. there's also a brief version.
'Diccionario etimologico castellano e hispanico', by Joan Corominas, 1980. this book is incredible! but it's not just a book, it's volumes of books.
I would totally recommend taking a history of the language class to anyone who is interested in Linguistics. you will learn more than you can ever imagine! I had a great Prof at PSU.
I love that people are pumped about linguistics!
take care!