There are literally hundreds of home study courses available for languages. Some of these are tried and tested, some of them have huge marketting budgets and some of them are just written by somebody who can speak the language with no knowledge of teaching at all.
First of all, let me just say that the old "listen and repeat" cassette and CD courses are the worst type of the language courses available. The manufacturers of these systems still do not seem to have realised that "listen & repeat" has never taught anybody how to communicate in a language. People who use them, at best, come away understanding what is said to them, but they can rarely formulate sentences of any substantial complexity required to do anything more complicated than order food from a menu.
Linguaphone* first pioneered this type of system but then learnt very quickly how poor they were. They have now greatly improved their systems and they now cover all 4 bases: reading, writing, listening, speaking.
Other courses, such as Hodder and Stoughton Teach Yourself....* prompt the student for much more varied and complex sentences which really help in the comprehension of the language. These courses are available as a book or a book & audio combination. Naturally the book and audio combinations provide considerably more learning potential than the book alone. There's nothing worse than arriving in a country fairly confident in the basics of the language, only to discover you can't understand the accent or pronunciation and they can't understand you either.
When choosing a home study language course, make sure that it tests you on creating complex sentences yourself, not just asking you to fill in the blanks of sentences. I think it is best to use a range of resources as no one course seems to cover everything. Don't just stick to a single brand or single author, try to mix and match at least two or three different home study courses.
There is no substitute, when learning a language, for complete submersion in the language. If you can go to a country that speaks your language, and more importantly, go to an area where English (or your native language) is not widely spoken, you will pick up the language a lot quicker than in your own country. If you can even spend time living with a family in that country, even better.
If this is just not possible or not feasible, a night college course should be a fairly cheap solution that, assuming it follows a reputable standard (GCSE/A-Level) should be quite an effective way to learn a new language.
There are also specialist language schools in your area that offer very good courses, but may be more expensive.
Either of these last two options provides the group learning that is another essential for language learning that you can't get from home courses. Home study courses can only get you so far, but talking with other people in the new language (rather than to a CD or to yourself) is much more effective.
If you have to use home study courses though, I would recommend buying at least 2 separate courses on the same language. No single home study course seems to be able to fully cover the basics of a language and I've never been able to put my finger on the cause of this. By simply adding another course froma different author and company, this seems to provide more than twice the comprehension and understanding. Maybe it's that different authors can explain the different nuances better than others, so the combination makes fora better understanding.
