Which book is best to improve vocabulary?

1. "Word Power Made Easy" by Norman Lewis - This is consistently mentioned as one of the top vocabulary-building books.

2. "English Vocabulary in Use" series by Cambridge - Available at different levels, these books provide structured vocabulary units with practice exercises.

3. "Oxford Picture Dictionary" - Uses images to teach words and provides context through stories and exercises.

4. "Merriam-Webster's Vocabulary Builder" - Introduces essential words with definitions, pronunciations, and example sentences.

5. "The Vocabulary Builder Workbook" by Chris Lele - Offers simple lessons and activities to teach over 1,400 must-know words.

6. "30 Days to a More Powerful Vocabulary" by Norman Lewis and Wilfred Funk - A popular guide to rapidly improve vocabulary.

7. Fiction books like "The Complete Sherlock Holmes" or "Harry Potter" series - Reading varied fiction exposes you to rich vocabulary in context.

8. "English Collocations in Use" - Helps learn common word combinations to improve fluency.

9. Classic novels like "To Kill a Mockingbird" or "The Great Gatsby" - Known for their evocative language and rich vocabulary.

10. "Verbal Advantage" by Charles Harrington Elster - A 10-step program to build vocabulary.

The best approach is often to combine dedicated vocabulary books with extensive reading of varied fiction and non-fiction. Consistent practice and usage of new words is key to retaining and expanding your vocabulary.