First up, kudos if you can name the song the title of this blog came from and who performed it.
Words are the stock in trade of a writer, but that doesn't mean you need an encyclopeadic knowledge of the complete 20 volume Oxford English Dictionary, or even the pocket edition. In the words of Stephen King, 'you have what you need'.
It never hurts, though, to have other words available and other resources that don't cost a fortune. Aside from a decent dictionary and thesaurus, which I hope you have, a book on idioms and phrases would be useful, but not essential. Go into any decent bookshop and you'll find quite a few, whilst secondhand bookshops will have even more. Don't neglect older versions either, especially if you're writing historical fiction as knowing how they used words 'back in the day' will help add to the realism of your book.
The internet is full of useful sites.
Wikipedia is much maligned, but the entries on language and science tend to be accurate as they are less contentious than biographies and history. Not free from contention obviously, but less so.
The Inky Fool is a great blog that has a new word every day, and the author has a book out at the moment, just in time for Christmas. Isn't that strange?
If you feel like paying OED a subscription you can have online access to their 20 volume edition and its supplements.
Dictionary.com is another useful site allowing you to find definitions of many words. It also has a word of the day, a thesaurus and several other gimmicky things you may or may not like.
Words may not come easy, but there are always methods and resources to improve our vocabulary and knowledge. Have fun exploring and share some of your own sites and books below.