Here are some books which really helped me to improve my teaching skills. It is important to continually improve because that is what it means to be a professional.

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Teaching Unprepared Students
This book really helped me because I thought my students would be just like me. However, this book helped me to see that it is the best students who typically become teachers. However, you actually teach good, average, and those students who struggle. This book will help you understand those who struggle.

McKeachie's Teaching Tips
This book is filled with teaching tips gathered over a lifetime of teaching. The tips are short but have a lot of meaning. So, it will likely take you years to read this book because you should just read 1-2 pages, then try to put the tip into practice, before going on to the next tip.