I have been trying to find some basic books to tackle grammar and word problems. I have found these two books to be extremely useful.
1. schaums Outlines - English grammar (ISBN 0-07-135985-0)
Deals with all the basics of grammar with exercises. This book will not address the kind of SC mistakes that we see on the GMAT but this I felt is a good book to start with and then move on the specific GMAT areas in Kaplan
or other guides.
2. How to solve word problems - Algebra (ISBN 0-07-134307-5)
Deals with all types of word problems and teaches in detail how to write equations in word problems. I found the kind of problems very similar to the ones we see on the OG and other guides. In this series they have many books for different topics like, calculus, trigonometry etc but the two I found most useful are the Algebra and Mathematics books.
If you are in the same situation as me (forgot all the basics in Maths and grammar) please take a look at these books.