Listmania!
If you are buying books about language
A Student's Introduction to English GrammarA Student's Introduction to English Grammar by Rodney Huddleston
Buy new: £12.79 / Used from: £8.96
You won't need another grammar textbook for English
The Cambridge Grammar of the English LanguageThe Cambridge Grammar of the English Language by Rodney Huddleston
Buy new: £109.83 / Used from: £109.85
The ultimate modern guide to English grammar, you might need a rich aunt for this one...
Merriam Webster's Concise Dictionary of English Usage (Dictionary) (Dictionary)Merriam Webster's Concise Dictionary of English Usage (Dictionary) (Dictionary) by Inc. Merriam-Webster
Buy new: £9.89 / Used from: £3.11
What writers actually write, rather than what they should write.
Penguin Writers' Guides: How to Write Effective EmailsPenguin Writers' Guides: How to Write Effective Emails by R. L. Trask
Buy new: £5.99 / Used from: £2.24
A modern guide to English usage
The Cambridge Encyclopedia of LanguageThe Cambridge Encyclopedia of Language by David Crystal
Buy new: £18.80 / Used from: £9.97
A wonderful introduction to the wonder of language, and its scientific study
Oxford Advanced Learners Dictionary (includes CD-ROM)Oxford Advanced Learners Dictionary (includes CD-ROM)
Buy used from: £16.63
This book is genius for language learners. Recommended over Collins et al.
Cognitive Linguistics: An IntroductionCognitive Linguistics: An Introduction by Vyvyan Evans
Buy used from: £44.62
Simply the best intro to one of the most exciting fields of linguistics - the cognitive enterprise!
The Minimalist Program (Current Studies in Linguistics)The Minimalist Program (Current Studies in Linguistics) by N Chomsky
Buy new: £28.45 / Used from: £19.50
Not exactly intro level, but required reading at some stage, and that better be before he gets onto his next book...