Most people who are studying foreign languages will have a whole library of books about their language of interest. However, there a few that you just cant do without. I show books for different languages but you can find the books I show in almost any language easily with the exception of my extremely small French only dictionary.
List of Books:
1. 501 verb books: When I was learning French I chose the 6 most common conjugations and learned the different verb types. Er, Ir, Oir .. I started by writing them hundreds of times each until I could recite them without thinking. Then I progressed to reciting them by memory. Next I created sentences and used all of the conjugations for each pronoun in each tense until is was second nature.
2. Standard Dictionary: Although there are lots of dictionaries out there very few do it well. Look for a dictionary with precise, up to date and adequate definitions. It also helps if they have examples of how the word is used contextually or idiomatically.
3. Theme Based Dictionaries: I personally really like Mastering Language Vocabulary by Barrons but Im sure there are others that are very good. The books gives you a lot of vocabulary words that are important in certain subjects but you may have not have thought of because they are more idiomatic or technical words that are uncommon.
4. Uber Tiny Pocket Dictionary: I found this dictionary while living in France and fell in love. I literally carried it with me everywhere I went for 1.5 years. At anytime I wanted to find out what a word meant that someone said in a conversation or while reading I instantly looked it up. This dictionary is French only so I only started using it after I was conversational. However, it is the most important book in my collection because it makes language learning convenient and immediate. Often times youll think of a word and then forget it because by the time you get home and can look it up youve completely forgot.
I hope this helps you get started learning your desired foreign language or improves your current studies.