The University of Leicester's Historical Directories web-site provides indexed facsimile images of many directories, including White (1850) and Pigot (I844) for Devon.
The Westcountry Studies Library has trade directories from 1783.
Direct Resources have surname indexes to 19th Century trade directories on-line; see their list of counties and directories. Note there are text and spreadsheet formats available. The Devon coverage is of towns and major villages in Slater's Directory of Devon 1852-3, White's Devon 1850 and Hunt & Co's 1848 & 1850 directories.
Billings, M. Directory and Gazetteer of the County of Devon. Birmingham (1857) 786pp.
White, W. History, Gazetteer, and Directory of Devonshire and the City and County of the City of Exeter , Newton Abbot, David & Charles (1968) 804 pp. [Reprint of the first (1850) edition of the earliest really detailed Devon directory] [Lookups(link sends e-mail)]
Pigot's 1823 Devon Directory. CD-ROM produced by S & N Genealogy (2000). [In Adobe Acrobat format]