There's a variety of ways to improve English language skills, including listening to podcasts, watching YouTube videos and practising having conversations with native English speakers. Still, textbooks and learning books remain the most reliable option, especially for more advanced learners who want to find out more about the nuances of the English language, take a systematic approach, and further develop and refine the skills required to write and speak grammatically correct English and make a careful, thoughtful choice of words.
Here're several books that can be especially helpful in this:
Practical English Usage, 4th edition by Michael Swan
Published by Oxford University Press, this textbook has sold over 2 million copies since the first edition was published in 1980, and for a good reason. It's comprehensive, clear, concise and well-structured, which makes it easy to find answers to the most common grammar and vocabulary-related questions and other useful information about the language.
English Made Easy, Volume One by Jonathan Crichton and Pieter Koster
This book is great for beginners who prefer a visual learning style. It focuses more on the actual use of the language and teaches students through pictures and practising vocabulary, conversational phrases and pronunciation.
High School English Grammar and Composition by H. Martin and P. C. Wren
Grammar is an English student's nightmare. This excellent book with easy-to-understand, clear explanations is useful not only for learners but also for everyone who needs a comprehensive, deep overview of English grammar rules.
30 Days to a More Powerful Vocabulary by Wilfred Funk and Norman Lewis
Learning new words is a never-ending process, and English in particular is very rich in vocabulary. Simple memorization more often than not can be a mind-numbing, tedious experience. Thankfully, this book is the very opposite of that. It will make your learning process more enjoyable and engaging and will help not only memorize but also retain new words.
99 Ways to Speak Better English Today by Clare Whitmell
This book isn't really a textbook, but more of a practical guide for busy learners who want to improve comprehension and speak English more like native speakers.
Strictly English: The correct way to write ... and why it matters by Simon Heffer
This book is written by an English historian, journalist, author and political commentator Simon Heffer and addressed first and foremost to English speakers who deeply care about the beauty, precision and elegance of the English language.