There are a number of books out there that can help you with your grammar. I used many of these during my years of teaching and then carried them forth to my other places of employment where they have been used for reference.
A comprehensive sort of "crash course" in improving your use of the language is "30 Days to Better English" by Norman Lewis. It explains how everything works and why--and in language everyone can understand. It may be out of print, but you should be able to pick up a used copy cheaply.
As far as texts go, the aforementioned Warriners has been used for ages. To many it the bible of English usage. Also quite good is "The Little Brown Handbook." A more contemporary approach can be found in a book titled, "Woe is I."
I can recommend all of these books wholeheartedly, but I cannot do the same for Strunk and White.
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