Many questions on this site have been about the etymology of a particular word or phrase. For English, resources like the Oxford English Dictionary often give researched etymologies full of details and citations. Are there any similar resources in Spanish for researching word etymologies? Are there any freely available online?
RAE's dictionary provides some information on etymology. Examples:
I think that's the most authoritative free online resource. There are others:
If you want a book, you might want to try Breve diccionario etimológico de la lengua española by Gómez de Silva Guido. It claims to have "10 000 artículos, 1 300 familias de palabras".
Besides the RAE itself, some of the other academy branches in the other Spanish speaking countries also publish dictionaries which can be very useful for regionalisms.
The Academia mexicana de la lengua publishes such a dictionary and it is also available free online: