[CAUTION: I am not a teacher:take the advice and or corrections offered in this post at your own risk.
If you doubt the information, please get a qualified opinion from one of the teachers on these forums.]
Before -blouen- and others go plunging into the first grammar book they can find, I would like to remind all learners that millions of people speak English perfectly well without having the faintest idea why the way they speak is "right" or "wrong".
They are native speakers, and they only consider grammar when they seem to be making mistakes and don't understand why, just as they only consult a dictionary when they cannot understand a word from its context. You wouldn't feel that reading the dictionary was more important than say...reading a book, for developing your reading skills, would you?
Well in the same way, I feel, reading a grammar book is much less important than having frequent conversations in the target language, preferably with native speakers if they are available.
To make a comparison, you don't have to have an understanding of thermodynamics or how to take apart the engine of a car before you drive one, nor would anyone suggest you could learn more about playing the piano by reading books on music theory than you could practising with a teacher.
I remind all readers as well that I am not speaking based on theory. I spent five years in a Latin classroom poring over grammatical rules, I never did manage to reach conversational ability, however within a year and a half of living among native speakers in China, and being observant, I could answer, or at least immediately understand the answers to most questions on a standardised test on Mandarin grammar.
In short, "driving" taught me how to "drive" much faster than reading books about it did. Speaking while referring to reference books when needed should no doubt help any learner to advance more quickly than reading first and doing later.