The 4000 Words book is one of the best, most comprehensive sources of English vocabulary and can even be used for self-study. The vocabulary in this collection is carefully selected based on detailed studies and covers approximately 95% of everyday conversations and even semi-technical English dialogues. If you're looking for a reliable and specific method to learn and expand your English vocabulary or are preparing for major exams like IELTS or TOEFL, this collection is undoubtedly essential.