Reference books aren't likely to be helpful to learn how to speak more natural English, and I don't think there's actually a way to teach natural English other than by speaking it with you. I think the best way for you to learn natural English (or any language) and the subtleties of it is to listen to immerse yourself in it - listen to music, watch television shows, and talk to people (like native speakers on this website) to get a feel for the language. It will be hard at first to make smooth conversation, but it'll get easier the more you speak yourself. Hope this helps!

Of course, if you definitely want some resource on grammar, there are many websites for that online. Here is one: http://www.englishlanguageguide.com/grammar/preposition.asp

Here is an example, and another web link which shows you the kind of brief lesson material that will give you confidence and knowledge both, in your English speaking and writing. There Once Were Two Cats of Kilkenny There once were two cats of Kilkenny, Each thought there was one cat too many, So they fought and they fit, And they scratched and they bit, Till, excepting their nails And the tips of their tails, Instead of two cats, there weren't any. http://www.mamalisa.com/?t=hes&p=1390 If you find a language partner to read over these kind of studies with you, it will be helpful. Or, you can join one of the study groups which seek members in the Discussion Forum of Italki Groups and individuals who have specific study materials like this, will quickly develop their English skills in speaking and writing.

Aftab: The best way to acquire a knowledge of general grammar is to being reading Nursery Rhymes. That way, your can form a memory of a correct grammatical construction in a natural way. Here is a link to one website that may help you. http://www.nurseryrhymes.com/ Also, you can listen to the Nursery Rhymes being read on YouTube.com and acquire a natural feeling for the construction of sentences and the correct NOUN & VERB Relation.