I would recommend the book -
IELTS - The Complete Guide to Task 1 Writing
It's very good as a self study book as there are many examples ranging from bar charts and tables to life cycles, processes and so on. For a teacher there are many examples to give students in the class or for homework.
available on Amazon
On Amazon USA it is rated 5 stars and 4.3 on Amazon UK. There are not many reviews there but here are a few.
Before taking the IELTS I was confident about it except Task 1 in Writing section. Therefore I purchased this book because it is devoted to only Task 1 and I wanted to concentrate my preparation time only on my weakest point - writing task 1. I got this book 2 weeks before the exam and read it twice. It is short and easy to read; it contains all necessary advice for Writing task 1. While writing a task 1 during the actual IELTS, I used all advice which I read in this book. I got a band 8 in IELTS' Writing section.
When I bought this product, I needed to gain additional practice in task 1, and I decided to have a crack at the title. In a nutshell,this book provides you with several examples of different types of reports, thereby making a contribution to the process of understanding of academic writing. After completing all exercises I realized that I can easily write a great report myself. It's very likely to score high with this source as I did in the writing section. The author should be encouraged to write the complete guide to the speaking part as well.