The path to mastering English vocabulary is paved with consistent reference and curiosity. Thanks to public domain works and generous educational publishers, a high-quality is just a few clicks away.
The full OED is massive (20+ volumes), but Oxford University Press offers an Abridged Student Edition PDF for free download on various educational repositories. It focuses on 50,000 of the most common English words, with concise definitions and example sentences.
Instead of carrying a heavy physical volume, you can store a comprehensive dictionary on your phone, tablet, or laptop.