Why does a book leaned up against a wall sometimes fall over after being stable for many hours? This problem has been vexing me for years. 
Sometimes, you lean a book up against a wall and it stays there (the static frictional force balancing gravity, etc), but then after a long time (hours or even days) it suddenly falls down without any obvious external perturbation (e.g. bumping the shelf). What causes this? 


*

*Is the book slowly slipping over time until it reaches an unstable
point and falls over? 

*Is it just being perturbed by some sudden jolt
that I can't detect myself (like a low-frequency vibration)? 

*Is it a combination of these two things? Something else entirely?

*Is there a name for this phenomenon? Is there research on it?

 A: It probably depends on multiple factors but one of the most important is how close the static friction force is to the maximum where you have pending or imminent slipping. At that point the book is unstable and the slightest movement, vibration, etc., will cause it to fall.
For the book the static friction force is a variable reaction force that keeps the book in static horizontal equilibrium, like at the foot of a ladder. But the static friction force  has a maximum possible value of $\mu N$ where $N$ is the force normal to the surface. If the book is initially leaning so that the static friction force is at or near the maximum the book will be unstable and collapse with the slightest vibration or movement.
The reason it might take several hours Is it may require the cumulative effects of imperceptible vibrations to cause the center of gravity to move and the maximum static friction force reached.
As far as I know no ordinary environment is vibration free. From a microscopic view  static friction is due to microscopic irregularities at the contacting surfaces causing them to interlock. Slight vertical oscillatory vibrations briefly separate the surface. A brief resulting net horizontal force in the direction away from the wall has the surfaces come back together to interlock microscopically further away from the wall slightly reducing the normal force and thus the static friction force. Over time the maximum static friction force is reached and the book falls
Hope this helps 
