Ideal for English Literature students is this one, generously placed in the public domain,by Hazel Hutchison (Royal Holloway College, University of London), Guide to Good Writing
For a book-length guide for students, I recommend Peck and Coyle's Write it Right
Books on Style
The resources above are aimed at the student market. Other books deal with the wider question of clear style, which is applicable to various academic and professional fields.
Strunk and White's Elements of Style is a classic older book. Today it perhaps seems a little dogmatic in its precepts, and it does not go far into he mechanics of how to be clear. But it is constantly engaging, blessedly short and inspires us to get rid of some of the clutter in our prose. A few pounds very well spent. The original 1918 text by Strunk is accessible here.
The best thing I have found on writing clearly is Style by Joseph Williams. His wonderful guide actually provides a course of exercises to help you get better (Strunk and White generally tell you get better). There are a rather confusing variety of editions of this book, which has gone through various publishers, but you won't go wrong with these two:
Style: Ten Lessons in Clarity and Grace contains the principles, with exercises
Style: The Basics of Clarity and Grace gives you the essence
Williams is commending a certain style, which one may call 'classic', characterised by the virtues of concision, clarity and balance. This is explored rather beautifully, with plenty of examples, in a book by Francis-Noël Thomas and Mark Turner, Clear and Simple as the Truth: Writing Classic Prose
That's more than enough to choose from. But just to round things off, last and least, here's a little handout of my own, largely drawn from Williams, with some practical experience thrown in:
And below is a brief summary of my own commandments. Looking at it again it seems rather hectoring, telling not showing, but maybe it's of some service as a memo. Good luck to all those coursework scribes out there.