A good bilingual dictionary is essential. There will be a million occasions when you read or hear a word that you don’t recognize and go running to your pocket dictionary. In extreme cases you can even communicate one word at a time with a dictionary. Just make sure your conversational partner isn’t driving at the time or he might not see the next bend coming. Better hope you’ve learnt how to yell ‘look out!’
You’re best off getting a light bilingual dictionary that fits in your pocket or shoulder bag. The Collins Gem dictionaries can be recommended in particular as they go into various contexts of the important words and you can learn just by browsing through them.
Serious students may want to get a larger, more comprehensive dictionary eventually, though, especially when it comes to recognizing slang.
Most teach-yourself language books have the same mentality of primary school teachers and seem to be more about how to waste your time rather than teach you that much. They’re filled with exercises that make you feel like you’re waiting for the lunch bell to go and are often accompanied by cutesy little cartoons that do little to actually communicate the lesson in question.
Basically you want to get yourself a nice and clear grammar book that goes systematically through all the important areas common to most languages and their idiosyncrasies. Get one that gives several examples of each aspect of the grammar in context and one that makes sense to you.
Also good are some of the books devoted to slang. These can give you a handle on street language but the problem with slang is that it often tends to be regional or restricted to a certain social class and situation. ‘What’s up, motherfucker?’ might go down well in Harlem but could be disastrous when offering condolences at a funeral. The same goes for other languages, too. But you can always ask locals for their take on the slang in question and they’ll give you an idea of if and when you can use it.
Remember the basic premise that spending money won’t make you learn a language and you’ll get the deal with the Linguaphone ads that promise you fluency in 3 weeks. Yes, if you were to study like a nut for 21 days, learning lists of vocabulary, writing, reading and listening to the language in every waking moment, you would indeed learn fast.
But paying $100 to listen to some patronising speaker repeat phrases over and over isn’t going to accomplish that for you.
You can read a book on the Kindle in your target language and with the right dictionary installed you can just touch a word to find out what it means. And it fits in your pocket. If you have large pockets.