I'm trying really hard and revising loads, but I haven't really read much around the subjects. Is it too late or is there extra that I can do from now?
Extra reading isn't required but it does help build your knowledge. Just go to a library and find a book or two related to what you're studying (this applies more for History than Englit) and read it.
But I really suggest practicing essays and learning to write quickly so you don't run out of time in exams.
You don't really need to do extra reading (not at AS anyway). I'd just practice essay writing for history - make loads of plans for exam answers and separate your answer into themes - political,social,economic etc,it makes it more coherent and organised.
English it's useful to get some text guides for the books you're studying - they give you some new perspectives on them which are interesting to the examiner,who see the same stuff repeated hundreds of time! You need to really make your answers unique and imaginative,not like everyone else.
No extra reading is needed, if you do edexcel in unit one just learn 'enough' content and focus on the exam technique (I went in not knowing much but because I knew the technique I got an A). Also, cross reference so much with the sources!
Unit 2 (if you're on edexcel) the content is so important, but do not waste your time with the minor things, learning acts/treaties etc. are helpful.