Real Academia Española. Diccionario panhispánico de dudas. PC 4460 .D5155 2005.
Deals with problems: common confusions, complexities and sources of error.
Also available online
Moliner, María. Diccionario del uso
Describes common usage for a comprehensive selection of Spanish words. For the English speaker, good for understanding nuances of usage beyond what is typically found in other dictionaries.
Vox diccionario de uso del español de América y España. Ref. PC 4625 .V69 2004.
Reflects contemporary common usage, including both Iberian and American sources. Definitions are identified by region or country.
Corominas, J. Diccionario crítico etimológico de la lengua castellana.  Ref. PC 4580 .C6.
Corominas, J. Breve diccionario etimológico de la lengua castellana.  Ref. PC 4580 .C58.
The standard work on the etymology, or derivation and historical development of words in the Spanish language. Emphasis primarily upon Castilian although some words of American origin are included.
Gómez de Silva, Guido. Elsevier's concise Spanish etymological dictionary. Ref. PC4580 .G65 1985.
“Containing 10,000 entries, 1,300 word families,” this dictionary is more current than Corominas, and would be a first choice over the latter’s Breve diccionario.
Sainz de Robles, Federico Carlos. Ensayo de un diccionario español de sinónimos y antónimos. Ref. PC4591 .S25 1967.
A Spanish-language thesaurus not unlike Roget's in English.
Batchelor, R. E. Using Spanish synonyms. Ref. PC 4591 .B38 1994.
This Spanish-language thesaurus is the most current we have, and includes English glosses and equivalents.
Vox. Diccionario de sinónimos. Ref. PC 4591 .V6 1977.
More concise and current than Sainz de Robles, it also features a dictionary arrangement similar to that of Webster's Thesaurus in English.
Caballero, Ramón. Diccionario de modismos (frases y metáforas) primero y único de su género en España.  Ref. PC 4460 .C2.
"Modismos" are sayings, proverbs, slang usages, and picturesque metaphors whose understanding is often difficult when simply translating word by word. Although quite dated, this work is still useful particularly for the English speaker who may have trouble comprehending "modismos."
Santamaría, Francisco J. Diccionario de mejicanismos. Ref. PC 4832 .S3 1974.
Includes words arising from or given distinctive expression in Mexican Spanish. Less prescriptive, more descriptive in orientation than the more Castilian works.
Jergas de Habla Hispana (Ref. PC 4971 .F57 2006)
Also available online.
Guide to current slang or argot in Spanish worldwide.
Diccionario de coloquialismos y terminos dialectales del espanol (Ref. PC 4971 .F58 2011)
Los que dicen ¡ay bendito! : dichos, modismos y expresiones del habla coloquial puertorriquena (PC 4854 .P8 N87 2001)
The online version of the 22nd edition (pub. 2001) of the Real Academia’s dictionary. Includes updates that will make it into the next edition, complete verb conjugations.Nuevo tesoro lexicográfico de la lengua española
Also known as the NTLLE, this prodigious lexicographic masterpiece of the Royal Spanish Academy allows the user to search all past editions of the Diccionario from 1726-1992. The results are given in full-image scanned directly from the pages of the dictionaries.Breve diccionario de mejicanismos
Published by the Academia Mejicana de la Lengua (the Mexican establishment of the Real Academia), this is a concise guide to uniquely Mexican words and usage.Wordreference.com: Online French, Italian and Spanish Dictionary
A multilingual dictionary including also a direct link to the Real Academia’s dictionary from within search results.Yourdictionary.com--Romance Languages--Spanish
The great dictionary of the English language, compendious, extensive, with complete etymologies, morphologies, histories of usage, containing hundreds of thousands of quotations illustrating usage.
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