These recommendations for preliminary reading are based on the four Part I papers, which all Linguistics Tripos undergraduates study in their first year. 

Sounds and Words

Crystal, D. (1997) The Cambridge Encyclopedia of Language (Chapters 27-29, 40). Cambridge University Press.
Pinker, S. (1995) The language instinct (Chapters 5 & 6). Penguin.
Ashby, M. & Maidment, J. (2005) Introducing Phonetic Science. Cambridge University Press.
Davenport, M. & Hannahs, S.J. (2005) Introducing Phonetics and Phonology. Hodder Education.
International Phonetic Association (1999) Handbook of the International Phonetic Association. Cambridge University Press.

Structures and Meanings

Aitchison, J. 2003 Words in the mind: An introduction to the mental lexicon Oxford: Blackwell. Third edition.
Baker, M. 2001 Atoms of Language Oxford: Oxford University Press.
Larson, R. 2010 Grammar as Science Cambridge, Mass.: MIT Press.
Levinson, S.C 1983 Pragmatics Cambridge, UK: Cambridge University Press
Mey, J. L. 2001 Pragmatics: An introduction Oxford: Blackwell. Second edition.
Pinker, S. 2007 The Stuff of Thought: Language as a Window into Human Nature New York: Viking
Tallerman, M. 1998/2005 Understanding syntax London: Arnold.

Language, Brain and Society

Crystal, D. 2004. The stories of English. London: Allen Lane (Penguin Books).
Harley, T. A. 2008 [1995]. The psychology of language: From data to theory. Hove: Psychology Press.
Holmes, J. 2001 [1992]. An introduction to sociolinguistics. London: Longman.
Mesthrie, R., Swann, J., Deumert, A., & Leap, W. L. 2000. Introducing sociolinguistics. Edinburgh: Edinburgh University Press.
Smith, N. & Tsimpli, I. M. 1995. The mind of a savant: Language learning and modularity. Oxford: Blackwell.
Yang, Charles. 2006. The infinite gift: How children learn and unlearn the languages of the world. New York: Scribner.

Linguistic Variation and Change

Crystal, D. 2003 [2001]. The Cambridge encyclopedia of the English language. Cambridge: CUP.
Crystal, D. 2002. The English language. London: Penguin.
Denison, D., and Hogg, Richard. 2006. A history of the English language. Cambridge: CUP.
Graddol, D., Leith, D. & Swann, J.. 2002 [1996]. English: History, diversity and change. London: Routledge.
Hughes, A., Trudgill, P. & Watt, D. 2005. English accents and dialects. London: Hodder Arnold.
Kortmann, B. & Upton, C. 2008. Varieties of English: The British Isles. Berlin: Mouton de Gruyter.
Song, J.J. 2011. The Oxford Handbook of Linguistic Typology. Oxford: Oxford University Press.
Van Gelderen, Elly. 2006. A history of the English language. Amsterdam: Benjamins.
Whaley, L. 1997. Introduction to Linguistic Typology. The Unity and Diversity of Language.  London: Sage Publications.