Germaniclang 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!
Germanic historical grammars, phonologies and (etymological) dictionaries available on the Internet
Abriss der urgermanischen Lautlehre : mit besonderer Rücksicht auf die noridschen Sprachen - Noreen, Adolf, 1854-1925
Here Old English = Middle English!
in which its forms are illustrated by those of the Sanskrit, Greek, Latin, Gothic, Old Saxon, Old Friesic, Old Norse, and Old High-German - March, Francis Andrew, 1825-1911
Being at the same time a historical grammar of the English language. And comprising Gothic, Anglo-Saxon, Early English, Modern English, Icelandic (Old Norse), Danish, Swedish, Old High German, Middle High German, Modern German, Old Saxon, Old Frisian, Dutch - Helfenstein, Jacob
Old English, ca. 450-1100 Word formation
Good to have if you can't come by Wordcraft by Pollington, Stephen. Pinner Middlesex England: Anglo-Saxon, 1993. Print. ISBN 9781898281023.
An etymological dictionary of the English language - Skeat, Walter W. (Walter William), 1835-1912. comp
chapters illustrative of the origin and growth of romance influence on the phrasal power of standard English in its formative period - Sykes, Frederick Henry, 1863-1917
Searchable Proto-Germanic, Gothic, Old English and Old Norse dictionaries. Click on the "Search" tab and make sure to follow the "Search help" link. NB you can search on modern English and modern German words!
Germanic philology (1913) by Loewe, Richard, b. 1863; Jones, John D
Germanische Sprachwissenschaft (1911) by Loewe, Richard, b. 1863
English translation elsewhere in this list
Germanische Sprachwissenschaft (1922) Volume 1 "Einleitung und Lautlehre" by Loewe, Richard, b. 1863
for courses in Chaucer, Middle English, and the history of the English language - Moore, Samuel, 1877-1934
Zipped LaTeX source
An introduction to Middle High German: being an outline of Middle High German grammar and... - Charles Pomeroy Otis
An introduction, phonological, morphological, syntactic, to the Gothic of Ulfilas - Douse, Thomas Le Marchant
Designed for use with Sisam's Fourteenth century verse and prose - Tolkien, J. R. R. (John Ronald Reuel), 1892-1973
A Middle-English dictionary : containing words used by English writers from the twelfth to the fifteenth century - Stratmann, F
Beiträge zur wissenschaftlichen Kunde der neulateinischen und der germanischen Sprachen - Diefenbach,
being a systematic account of Old English vowels and consonants and their correspondences in the cognate languages - Mayhew, Anthony Lawson, 1842-
Übersichtstabellen zu Lautentsprechungen und zur Kasusbildung des Nomens und Adjektivs im Germanischen (1910) by Wood, Francis A. (Francis Asbury), 1859-* Urgermanisch, Vorgeschichte der altgermanischen Dialekte (1913) by Kluge, Friedrich, 1856-1926
Utkast till föreläsningar i urgemansk judlära : med huvudsakligt avseende på de nordiska språken till den studerande ungdomens tjänst (1890) by Noreen, Adolf, 1854-1925
In Swedish. Hopefully someone besides me (BPJ) can read it! ☺
A Proto-Germanic dictionary from the Germanic Lexicon project. The reconstructions are hopelessly dated but with a caveat emptor still good for conlanging, and the cognate lists are entirely good!