How to Ace the English Language iGCSE is available in Kindle format or paperback, and neither costs more than a tenner.
There are plenty of examples there of good practice, tips for revising, and ways to improve your writing in advance of taking the exam.
Buy it here from Amazon: http://tinyurl.com/hr8bmyl
Don't forget that I still run live crammer courses, the courses which inspired the book. What would it add to your child's study to attend the crammer if you already own the guide? New examples, for one. A chance to have homework and questions marked by me, with my examiner's eye. And some things that I can't print in the book, but I'll tell you off the record. The next one is scheduled for April 2017. See "Upcoming Courses" tab.