An interesting video lesson by Bradford Werner
In this video, Bradford Werner talks about the right hand thumb position and the related exercises.
When adjusting your hand and thumb position remember to also consider your guitar position, footstool height, and all the other variables.
In the video Bradford also mentions a famous book, The Bible of Classical Guitar Technique by Hubert Käppel, a 246-page course book that contains many technique exercises (arpeggios, scale exercises, hammer-ons and pull-offs with the left hand, tremolo, flamenco techniques, speed training, vibrato techniques and dynamic exercises).
It can be easily found on Amazon.