All about Euler angles and rotations, by StarManta:
https://starmanta.gitbooks.io/unitytipsredux/content/second-question.html
Notes on clamping camera rotation and NOT using .eulerAngles because of gimbal lock:
How to instantly see gimbal lock for yourself:
Also for future reference, if you post a code snippet, ALWAYS USE CODE TAGS:
How to use code tags: Using code tags properly