International Encyclopedia of Linguistics (Oxford)
Encyclopedia of Language & Linguistics (Elsevier)
Both encyclopedias are good for looking up linguistics concepts as well as reference information on languages and language families.
Encyclopedia of Applied Linguistics
Also good for foundational articles when your topic is applied (e.g. second language learning, interpreting, speech therapy...).
Atlas of North American English: Phonetics, Phonology, and Sound Change Reference Atlas Stand
E-book version of the Atlas
Focuses on phonological differences in North American speech
Watch out for variant spellings and alternate names, especially in the names of languages.
Example: The Ewe language is also known as Ebwe, Efe, Ehwe, Eibe, Eue, Eve, Gbe, Krepe, Krepi, Popo, or Vhe. I found this alternate name list in the Encyclopedia of Language and Linguistics, a good source for such lists.
Use a * to cut off a word at its stem, e.g. diglossi* retrieves diglossia and diglossic.
Some search systems, including ProQuest (used in LLBA) and the ZSR homepage, will return matches to all the words you entered, regardless of word order or words in between. In EBSCOhost (used in MLA), you need to type the word AND (or use the second row) if you want to enter more than one word that is not an exact phrase.
If you can't use a star to search for spelling variants, use OR: swiss OR switzerland
Putting it all together: diglossi* AND (switzerland OR swiss)
Too many irrelevant results? Try limiting by date, language of article, or to peer reviewed (scholarly) articles