Varieties of English
The Handbook is by far the most thorough reference work on phonology and the first-ever comprehensive overview of the morphology and syntax of varieties of English in the world. The genuine speech samples and interactive maps of the online version not only supplement the printed articles by offering lively illustrations of the varieties of English around the world, but the material offered can also be used for linguistic research.
Atlas of North American English : phonetics, phonology and sound change, 2006, Mouton de Gruyter
Ref. Atlas PE2808 .L26 2006
Also available online.
Cambridge encyclopedia of language 3rd ed, 2010, Cambridge University Press
P29 .C64 2010
This does not look like a traditional encyclopedia since much of the information is in outline form. However, it does provide lengthy overviews on each of the world's major languages, including the language's evolution, the population who speaks it, and something of its geographic distribution. Linguists and other researchers will appreciate the uniform entries on all aspects of the language's script, phonology, morphology and syntax. There is an appendix of scripts in volume 2, as well as a lengthy bibliography.
Encyclopedia of Language and Linguistics 14 vols, 2005, Elsevier
Ref. P29 .E48 2005
Detailed and well-organized articles provide clear information on most every aspect of linguistics. Bibliographies are included. Volume 14 provides a list of the major languages of the world and language maps as well as a subject index.
Summer Institute of Linguistics Glossary
This online glossary includes both definitions and illustrative examples of linguistic terms and concepts.