Grammar is important. Having good grammar skills helps you not only to express your ideas clearly but also to leave a good impression on the reader.
Whoever you are, having good grammar skills helps you write better sentences.
Whether you are studying or working, having good grammar skills will always work to your advantage.
For beginning learners, Basic Grammar Step by Step is a good book to have. Written in simple English, the book uses a systematic approach to teach basic grammar. Grammar terms are defined and grammar concepts introduced in sequence. When you finish this book, you will have a good grasp of basic grammar.