If you have studied English Language before, you will already be familiar with the way language is described as well as some of the areas of study.
A stimulating introduction to the subject is Steven Pinker’s book: The Language Instinct (Penguin, ISBN: 9780141037653).Depending on your particular area of interest, you might also want to read:
Aitchison, J. (2007) The Articulate Mammal: An Introduction to Psycholinguistics, Cambridge University Press ISBN: 0415167914
Allen, K., Bradshaw, J., Finch, G., Burridge, K. and Heydon, G. (2010) The English Language & Linguistics Companion, Palgrave MacMillan ISBN: 9781403989710
Crystal, D. (2003) (2nd edn) The Cambridge Encyclopaedia of the English Language, Cambridge University Press ISBN: 0521559677
Pinker, S. (2008) The Stuff of Thought: Language as a Window into Human Nature, Penguin ISBN: 9780141015477