Hi. I am not familiar with RS Agarwal but the Wren and Martin book is supposed to be good. I was pretty solid on grammar in high school, so I don't have that book, although I do have one called Grammar Smart, which is pretty good.
The GMAT prep books have pretty good reviews of math formulas and grammar rules, too. Have you bought any yet? I've been out of school for a while, so I got Math Workout and Verbal Workout from Princeton Review
in addition to the regular guide. You can get them from Amazon and have them shipped.
Hope this helps...very smart of you to plan ahead so well!!