This work traces the etymologies of the entries to their earliest sources, shows their kinship to both Spanish and English, and organizes them into families of words in an Appendix of Indo-European roots. Entries are based on those of the Diccionario de la lengua española de la Real Academia Española.
Rarely do we find a Spanish dictionary with etymology as we're so used to when we only consult English dictionaries. Spanish is by far the most taught language in the US and one of the most in many countries in the world, which makes lack of etymological dictionaries in the market an ostensible hole. Before I found Dr. Edward A. Roberts' dictionary, I had been using Guido Gómez de Silva's "Breve diccionario etimológico de la lengua española" and the online Wiktionary. While Dr. Silva did a very good job and his book used to be either the only or clearly the best such dictionary, it has left out some words in order to be "breve" or brief. Dr. Roberts' book is an excellent contribution to this field, serving as both a complement to and a replacement of Dr. Silva's "Breve diccionario", and as a bonus, the book is in English, if Spanish itself is still a language hurdle to the reader.
Etymology is not academic study only scholars care about. It's also a tremendously helpful tool in vocabulary learning, perhaps except for young children, who prefer rote memory instead. For example, to remember "seguir", a cognate "sequel" can be found by looking up the etymology of both words. A less obvious one is "corazón". Once you know it's cognate with English "core", the word is not that hard to remember.
These two-volume books are highly recommended for adults learning Spanish, if you prefer meaningful association to rote memory, and of course for those that just wish to have a dictionary on hand that provides rich history of this beautiful language as well as its culture. Although the same information is sometimes available in other books (such as Dr. Silva's book) or online (such as Wiktionary.org), none is more complete as this "Comprehensive" dictionary.