This is one of the best and recommended book for IELTS Academic Writing Task 2. The book contains more than 400 Solved Essays which are often asked in the exams. The beginning section of the book also talks about different types of essays that come in IELTS exams and how to attempt them.
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http://usafiles.net/11gI/IELTS_Academic_Essays_from_the_Past_Exams_-_400+_solved_essays.pdf