Romlang reference works online
Most of these links are to digitized, somewhat dated texts which have passed into the Public Domain and been uploaded to the Internet Archive, which are still perfectly good, or perhaps because of their datedness even better, as fodder for conlanging. They are offered in many file formats. Follow the "All Files: http" link to download. The b/w PDF, if available, is best for printout. The OCRed .txt version is full of errors but still good for searching!
Romanic historical grammars, phonologies and (etymological) dictionaries available on the Internet
Archivio glottologico italiano
Several yummy volumes at the Internet Archive. Italian isn't hard if you know Latin or both English and French!
The cursus in mediaeval and vulgar Latin (1910)
Author: Clark, Albert Curtis, 1859-1937; Oxford Philological Society Bound with the author's Fontes prosae nvmerosae. 1909
Diccionari de l'Institut d'Estudis catalans
Searchable dictionary of Catalan, including etymologies (in Catalan)
Diccionario de la lengua española
The Real Academia Española's searchable online dictionary, including etymologies (in Spanish)
Dicţionarul explicativ al limbii române
The Romanian Academy's searchable online dictionary, including etymologies (in Romanian)
Dictionnaire étymologique de la langue française (Clédat 1914)
Somewhat dated French etymological dictionary. Find headwords in the 'fulltext' with grep using beginning-of-line anchoring and a capital first letter in the search word!
Einführung in das Studium der romanischen Sprachwissenschaft (Meyer-Lübke 1901)
An introduction to Romance linguistics (in German).
Grammaire des langues romanes; (Volume 1) - Meyer-Lübke, Wilhelm, 1861-1936
The French translation of a thorough German treatment of Romance historical phonology and grammar.
Grammaire des langues romanes; (Volume 2) - Meyer-Lübke, Wilhelm, 1861-1936
The French translation of a thorough German treatment of Romance historical phonology and grammar.
Grammaire des langues romanes; (Volume 3) - Meyer-Lübke, Wilhelm, 1861-1936
The French translation of a thorough German treatment of Romance historical phonology and grammar.
Grammaire des langues romanes; (Volume 4) - Meyer-Lübke, Wilhelm, 1861-1936
The French translation of a thorough German treatment of Romance historical phonology and grammar.
A grammar of the Spanish, Portuguese, and Italian languages (1815) by Woodhouse, Richard
intented to facilitate the acquiring of these sister tongues by exhibiting in a synoptical form the agreements and differences in their grammatical construction.
Grammatik der romanischen Sprachen (Volume 1) - Meyer-Lübke, Wilhelm, 1861-1936 Vol. 1. Lautlehre
Romance historical phonology by one of the German heavy dudes.
Grammatik des Altfranzösischen (Laut- und Formenlehre) (1893) Schwan, Eduard, 1858-1893
Historical phonology and grammar of Old French (in German).
A historical French grammar (1899)
Author: Darmesteter, Arsène, 1846-1888; Muret, Ernest, 1861-; Sudre, Leopold Maurice Pierre, 1855-.
A historical grammar of the French tongue. (1888)
Brachet, Auguste, 1844-1898; Kitchin, G. W. (George William), 1827-1912.
Historical primer of French phonetics and inflection. (1900)
Author: Brittain, Margaret S (1900), With introductory note by Paget Toynbee (1900).
Historische grammatik der französischen sprache (1908) vol. 1
- Meyer-Lübke, Wilhelm, 1861-1936. Internet Archive: Free Download.
Historische grammatik der französischen sprache (1908) vol. 2
- Meyer-Lübke, Wilhelm, 1861-1936. Internet Archive: Free Download
An introduction to old French (January 1, 1896)
Author: Roget, Franccois Frédéric, 1859-.
An introduction to Old French phonology and morphology (1909)
by Luquiens, Frederick Bliss, 1875-1940.
Introduction to Vulgar Latin (1907) by Charles Hall Grandgent
Somewhat dated but a Romconlanger must-have!
N.B. The other version at the Internet Archive, previously linked here lacks pp. 160-167!
Lateinisch-romanisches Wörterbuch - Körting, Gustav Carl Otto, 1845-1913
At the Internet Archive. I have no idea how this compares to Meyer-Lübke, but I guess the more the merrier! Another version is here. The same remarks for download and search strategies as for Meyer-Lübke (and any book at the Internet Archive) apply.
mesPledaris
Searchable dictionaries of the Rumantsch dialects, in German, English, French and Italian.
Novum glossarium latino-germanicum mediae et infimae aetatis :
Beiträge zur wissenschaftlichen Kunde der neulateinischen und der germanischen Sprachen - Diefenbach,
An outline of the phonology and morphology of old provençal (Grandgent 1905)
A no-nonsense description of a facinating and historically important Romance language by the great American scholar.
Outlines of French historical grammar : with representative French texts (1899)
by Baker, Alfred Thomas, 1873-1936.
Romanisches etymologisches Wörterbuch
by Meyer-Lübke, Wilhelm, 1861-1936
The one etymological dictionary of the Romance languages to have at last on the Internet Archive! Be aware that this is an early (first?) edition and so not authoritative by scholarly standards but heavenly honey for a romconlanger. Beats your Latin dictionary any time and is a must-have for the Romconlanger even if you don't know German! Arm yourself with a (preferably somewhat dated! ☺) German dictionary and a heap of Romance language dictionaries, or their online equivalents. I recommend following the All Files: HTTP link to get to the right place to download the stuff. Of course the b/w PDF version is best for reading. I recommend the.txt version for quick searching (yay grep!) even if the OCR is crappy, to find out what to look for in the graphical version.
Rumantsch Grischun per Rumantschs
A brief pdf grammar from the Lia Rumantscha outlining the morphology of Rumantsch Grischun (in Rumantsch Grischun)
Vocabolario Etimologico della Lingua Italiana
Searchable scanned images of Vocabolario Etimologico della Lingua Italiana di Ottorino Pianigiani with Romance cognates
WordReference.com
Online multilingual dictionaries notably including Rumanian, Italian, French, Spanish, Portuguese to/from English. Use the query box with the drop-down menu at the top of the page, not the big form with radio buttons which accesses fewer languages!