Video Lesson of the Week: Right hand thumb position and exercises for classical guitar. I also cover a left hand exercise near the end of the video. You can see more lesson videos at the lesson archive page. The book I mention can be found on Amazon: The Bible of Classical Guitar Technique by Hubert Käppel which is an amazing 245 pages of exercises and text. When adjusting your hand and thumb position remember to also consider your guitar position, footstool height, and all the other variables. You can see more lesson videos at the lesson archive page or my list of technique books.
Video Link & Source: https://youtu.be/BpU1zYKp404