If you are learning english then you have to know the vocabulary more and more to control and command on english in writing as well as talking , then in other hand if you dont know the many words and you have to improve your english within a month then , I suggest you to go with oxford dictionary because it contains more than sufficient words to command on english and most imoprtant thing is that, there is a pictorial representation of many words so that you can learn very quickly and you can not forgot easly.
sometimes you habe to learn things like story wise then some of the words in this dictionary are also related with story for the betterment of your knowledge and to improve your skill, by the help of this dictionary you can also teach your cousins or any else.
The vocab provided by this dictionary is superb and this will hepl to growth of english in future too.