A writer uses many tools: pencil, pen, paper, computer, an imagination and a good dictionary and thesaurus.
The Oxford English Dictionary is an unabridged dictionary based on historical principles and answers questions about English spelling, grammar, usage and origin. The OED is also a record of the English language and provides timelines of when words entered the English language. It is considered the longest and most complete official dictionary.
There is also an OED Thesaurus. This companion provides aid choosing the correct word and meaning based on currency and style, synonyms, metaphors, and organized information on the history of the words.
On Amazon you can purchase an unabridged 20 volume set of the OED for more than $1200 and a two volume set of the OED thesaurus for $500.
However, you can also find a free online version of both: http://oxforddictionaries.com/
The use of words is the most important skill a writer can have. Therefore, you must have access to one of the best “word” resources. Good luck and happy writing!