If you have read my previous articles, you have picked up on my emphasis of needing written material to accompany any foreign language learning experience. However, Not all books are created equal, and there are many styles to consider adding to you collection.

General Textbooks

Think of the general textbook as the book you learned from in school. If you took a foreign language in high school or college, you will no doubt have a good idea of what a language textbook looks like. In keeping with tradition, textbooks are generally boring and light on details - and you won't find any new inventive avenues explored in them, despite the fact that a new one is published every year. That's fine, however, if you are learning a language on your own and are simply supplementing an audio or computer course with a textbook as reference material. Most standard high school or college texts will have sufficient reference material (grammar, vocabulary and glossary) to help get you through an audio program. The other upside is that you can usually find old textbooks for next to nothing at garage and yard sales

Side By Side Stories

As far as an active learning book goes, nothing beats side-by-side books/web news sites. The concept is simple, on one side of the page is the English version, and the other side is in the new language. There are three styles of this type of book - the literal translation, the paraphrased translation, and native interpretation. Native interpretation is the best, where the language story is told, as it would be in the native language, next to the English version. This helps the English speaker to understand structural differences between the languages, as well as the omission, or inclusion of different words. The other types are not quite as valuable, but are still useful, especially since these types of books are not available in all languages (take what you can get). I recommend picking up at least one of these, if you can find it in the language you are learning.

Grammar Books

While all textbooks and general language learning texts cover grammar concepts, there are few if any relating the new grammar contexts to your native language. What's more problematic is that deficiencies in learning a foreign language are largely due to lack of understanding your own native language's grammar constructs. This type of book is invaluable, and I know of only one series that accomplishes the task - the "English Grammar for Students of the X language" series. I recommend these books to supplement any, and I mean any foreign language courses on the market.

Workbooks

Finally, language workbooks are an excellent way to round out your writing skills in your additional tongue. I am specifically referring to books that ask a question in the new language and require a response in that language. This not only helps improve grammar skills, but also rounds out your understanding of verb tenses and pronoun use. Along the way, vocabulary is also improved as subjects are varied to keep the lessons interesting.
Workbooks are also great when working with new alphabets and scripts - so I would highly recommend adding a workbook for anyone learning Russian, Chinese, etc… Learning to write in a new language provides excellent reinforcement of all other concepts taught in audio and computer lessons.

In Summary

Most people, including myself, cannot learn languages using only written material. On the other hand the effectiveness of audio (and various computer courses) will increase dramatically when coupled with any sort of visual representation of the words your hearing. Best of luck in your search for the best books!