Yes, your dictionaries. You do not really believe anyone can master the English language, do you? Learning English is a lifelong effort, and among the indispensible tools for that permanent study are good dictionaries, of which there are many.
The most comprehensive English dictionary is The Oxford English Dictionary, aka the OED, a twenty-volume, 22,000-page reference on English. If you find that as intimidating a format as I do, I suggest The Compact Oxford English Dictionary, which photoreduces the entire OED down to just 2,416 pages; it includes a magnifying glass, but I find I can read the fine print without it. If you find even that daunting, I can recommend Oxford English Dictionary Online. I cannot recommend the CD-ROM version of the OED, since the software was written to emphasize the protection of their intellectual property over usability to the point of inconvenience and unreliability, according to most of the user reviews on Amazon.
The Chicago Manual of Style recommends you use two dictionaries: Webster's Third New International Dictionary (book and CD-ROM) and its more frequently updated abridgment, Merriam-Webster's Collegiate Dictionary (book only; a CD-ROM version is available separately). Webster's Third New International is far more complete than Merriam-Webster's Collegiate, but the Collegiate is updated more often, which is why Chicago recommends both.
You need not own these dictionaries, but you need some dictionaries, and you should explore them. Our words influence our ideas, or lack thereof.