Why does the hard drive keep crashing on 2009 MacBook Pro?

Question

We are now on the third hard drive, the second in 5 months.  The problem started after I dropped book on it, but no repairmen have suggested that has anything to do with it.

Answer 1

drop your books on it and you can crash your armature heads in the HD, ....HD are fragile.
I assume the first one died of age.....second from the book drop.......and the third? 
endlightend 
Hard drives don't fail often, but they do occasionally
They fail, and often, and new ones fail a LOT more than older ones (sounds silly, but logical)
Hard Drive Warning (all makes and models)
Ironically but logical, new hard drives are far more fragile than one that has been working for several months or a couple years. So beware in your thinking that a new hard drive translates into “extremely reliable”!
Hard drives suffer from high rates of what has been termed "infant mortality". Essentially this means new drives are highly prone to failing in the first few months of usage. This is because of very minor manufacturing defects or HD platter balancing, or head and armature geometry being less than perfect; and this is not immediately obvious and can quickly manifest itself once the drive is put to work.
Hard drives that survive the first few months of use without failing are likely to remain healthy for a number of years.
Generally HD are highly prone to death or corruption for a few months, then work fine for a few years, then spike in mortality starting at 3-4 years and certainly should be considered end-of-life at 5-7+ years.
The implication of this is that you should not trust a new hard drive completely (really never completely!) until it has been working perfectly for several months.
Given the second law of thermodynamics, any and all current mfg. HD will, under perfect storage conditions tend themselves to depolarization and a point will be reached, even if the HD mechanism is perfect, that the ferromagnetic read/write surface of the platter inside the HD will entropy to the point of no return for data extraction. HD life varies, but barring mechanical failure, 3-8 years typically.