Hi again Tom,
Although grammer is not so important in English, but it's important to be skilled at it..
I think the first step you should do is testing your level, for example if it's advanced so you should buy advance grammer book that will help u a lot...so it all depends on your level..
In my case I'm using an interesting grammer book, that really helped me a lot, it's: "The advanced grammer book" by Jocelyn M.Steer and Karen A.Carlisi..
And we should keep in mind that practice makes perfect, learning something without practicing it is really a waste of time..
That's all what I can say.
Hope I helped.