I recommend these texts for additional study.
You can access these texts safely and for free at the Internet Archive.
There are PDFs available for some.
- Click on the links below.
Cullen, Pauline. Vocabulary for IELTS with Answers. Cambridge University Press, 2008.
Cullen, Pauline. Grammar for IELTS with Answers. Cambridge University Press, 2007.
Cullen, Pauline. Grammar for IELTS Advanced with Answers. Cambridge University Press, 2012.
Swan, Michael, and Catherine Walter. The Good Grammar Book. Oxford University Press, 2006.