King's graduate Zadie Smith (English, 1994-1997) celebrated and defended local libraries in this 2012 essay, explaining that:
"Well-run libraries are filled with people because what a good library offers cannot be easily found elsewhere: an indoor public space in which you do not have to buy anything in order to stay."
As your exams come to an end and a lovely, long Summer beckons, you'll have more time to read around your subject. If you don't already use your local library, you will find out where it is and what it has to offer here. If your local library doesn't have what you're looking for, you can request an inter-library loan.
Your local university library may be able to help, too. For example, Newcastle University's Sixth Form Access Scheme provides reference facilities for Year 12s and 13s in the North East of England.