There are probably more fillable gaps in anyone's languge arts knowledge than any other area of study. Language is constantly evolving, changing its face and sometimes even its innards to accommodate the dictates of the Academy and society. Who can keep up with it all? who even has enough foundational knowledge to know what all those changes are?
Keeping up with all the trends isn't necessarily the point, though. The ultimate aim of all language arts instruction is communication: for the student to be capable of communicating, and able to be communicated to. As far as that is accomplished, language arts education has been successful; that doesn't mean there isn't always room for improvement.
It also doesn't mean that there aren't reference works you should keep around at all times. Everyone needs a good dictionary; when you encounter words you know, how else are you going to know what they mean? You could just head to the Internet, but there's so much misinformation there, having a hard copy of Webster's is a much safer (and more reliable) bet.
A compact but comprehensive thesaurus is also important. Here's a word of advice: don't overuse it. Roget's Thesaurus has lots of words you've never heard of, and while sometimes they turn out to be exactly the word you're looking for, it's not a good idea to throw million dollar words into your writing if you're unfamiliar with them.
The two best uses for a thesaurus: using it to remember a word you already know, or scanning it to build your vocabulary when you aren't writing. A good cook doesn't use spices she's never heard of just to make food interesting, and a good writer doesn't throw around strange verbiage just to make people think he's smart, or because he's already overused a simpler term.
Much of what you'll find here is designed to help with spelling, expand vocabulary, or assist writing. Everyone has particular strengths and weaknesses, and you'll have to ultimately decide what your students (or you) need. For specific elements of the language arts (grammar, rhetoric, phonics, etc.) take a look at our subject-specific resource sections; the books here are pretty broad and reference-oriented.