You may try Cambridge's "Advanced Grammar in Use" or Oxford's "Practical English Usage". As aguide to clearer and more fluent writing - Sir Ernest Gowers, "The Complete Plain Words". I think English learning resources such as Cambridge, Oxford or BBC are the most reliable.
I also found this blog post on the chepest textbooks to get online (smart.study/blog/best-10-sites-to-buy-cheap-textbooks-online/), in case you will need one. Good luck!
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