"Japanese for Busy People" is an excellent book, as it breaks things down and tells you when to use it and why it's used, etc. There's also "Japanese for Young People" (made by the same company), which teaches the reader by example. The latter is usually best when being taught by a native Japanese teacher, the former is good for self study. I also have "Japanese - Comprehensive Grammar" which deals with advanced grammar, speech patterns, etc. I also like it because it teaches you how Japanese really speak (i.e. non-textbook style), and it shows many examples, which for me is key. I highly reccomend learning textbook (formal) style first though.