Korean is a relatively modern language which was formed in the 15th century. It originally borrowed the Chinese script but has since developed its own. There are 78 million speakers of Korean in the world and it is consequently ranked 13th in terms of its number of total speakers. It is the official language in North and South Korea; as well as a very small part of China.
So can you really just buy a book and expect to become fluent?
There are many self teach guides on just about any subject you care to name from cookery to martial arts. You will have no trouble finding a book on the Korean language and a comprehensive book shop will almost certainly have at least one. There are a number of advantages to using this method and it can have some fast results.
Firstly, you can learn at your own convenience: unlike attending a class, you don't have to read a book at a certain time and you can easily pick it up when you have ten minutes to spare on the train or lunch hour. In this way you can continuously learn the language.
Many of these self teach books also contain tests and revision chapters to help you monitor your progress and pinpoint any weak areas that you may have developed. In this way you can go back and revise any areas of weakness and make sure you learn the basics well, before moving on to the next chapter. The books are generally very well organized and offer a progressive learning structure in which each chapter introduces more and more complex ideas and grammatical aspects of Korean.
I would suggest that you bought a copy which comes with a CD as this will allow you to hear the words and practice some of the listening activities as well as just reading the book. You need to understand how to pronounce the words and be able to understand what somebody may be saying to you. If you use the CD then you are likely to have more rounded language skills.
In terms of cost effectiveness there is very little that beats a book and they often come with online resources available on their websites. You can supplement your learning by opting to read some online contents as well to gain a broader set of skills and a much wider vocabulary.
Self teach guides are a good way to pick up new skills but are nothing if you don't practice. You also need to make sure that you pick up the book once or twice a day if you are going to retain what you have learned. If you have the willpower and motivation that's required then you will be ready to buy the intermediate and advanced editions in not time at all. Just keep up the reading and make sure you keep yourself engaged and interested – there really is nothing more to it.