These books are another favourite of mine. Essentially you get 4 full IELTS papers once more produced by Cambridge the co-owners of the IELTS test. What I like about them is that there is zero 'fluff'. You simply get 4 IELTS papers to practice with and have a go at. There are model answers/sample student answers in the back of these books and these are the main way in which you can gain a clear understanding of what you need to do in order to score well in the IELTS test, as each sample answer is then given a band score with the reason why it achieved that band, great for learning from.
They have also moved to using QR codes so that you can download the audio for the Listening tests, example Speaking test video, answer keys with extra explanations and sample Writing answers, so it is nice to see they are moving with the times! The only real draw back of using these books and the same with any IELTS prep book is that ideally you are still going to need to get feedback from a 'real person' to check how well your English is going and what mistakes you are making, but as far as IELTS books go, this is hard to beat. Be aware that there are separate version for the IELTS General and Academic formats of the test and you will need to purchase the correct version for yourself as the tests are very different in places. Click here for General and click here for Academic.