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Ultimate English Language Reference list
The Oxford English Dictionary : second edition (20 Volumes Set)The Oxford English Dictionary : second edition (20 Volumes Set)
Buy new: £427.50 / Used from: £497.27
The ultimate etymological dictionary, probably due to become extinct in hard (paper) copy. Lists all recorded words from Norman Conquest to present day. Consider also the Shorter Oxford or the CD Rom.
Roget's International ThesaurusRoget's International Thesaurus by Barbara Ann Kipfer
Buy used from: £17.65
Bearing the name of the original mass produced thesaurus (the first Roget's Thesaurus being published over 150 years ago) this is one of the best on the market.
A Comprehensive Grammar of the English Language (General Grammar)A Comprehensive Grammar of the English Language (General Grammar) by Randolph Quirk
Buy new: £150.49 / Used from: £115.54
The most comprehensive account of English Grammar ever written, with a level of detail that could suggest an anal fixation. Consider also the student edition.
Fowler's Modern English Usage (Oxford Language Classics)Fowler's Modern English Usage (Oxford Language Classics) by H.W. Fowler
Buy used from: £11.28
The most (in)famous prescriptive guide to the use of English. Not to be taken too seriously, but Fowler is an authority to the 'correct' use of English, spoken and written.
The Cambridge Encyclopedia of the English LanguageThe Cambridge Encyclopedia of the English Language by David Crystal
Buy used from: £16.22
Comprehensive introduction to the study of the English language, providing a readable account of almost every conceivable perspective on the English language.
The Cambridge Encyclopedia of LanguageThe Cambridge Encyclopedia of Language by David Crystal
Buy new: £23.74 / Used from: £12.43
By the same author, but this time stepping away from the focus on English, looking at language as a whole.