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As far as my area of the world (SE Michigan) I don't know of any police agency that has the extra man/woman to send to the library to protect the books? Saginaw, a city of about 70,000 just laid off a lot of officers.
Budgets in Michigan aren't working the way the manager's (bad politicians) had thought they would. Detroit, a much larger city is also dealing with big budget crunch too. I might add, on instances when a detective or off duty officer would be at a library, he or she could get involved in something. Our big problem these last two weeks are murder. Saginaw has had 8 killed in the last 6 days. Flint and Detroit are also hot areas too. Drugs are in one way or another almost always the root of the issue. Someone said to me the other day to look at the bright side.......The thugs are killing each other off. Too bad many stray bullets find citizens that were minding their own before being shot. Thursday a armed robber entered a Flint party store. He was dressed in black with a mask on as well. He fired a shotgun at the clerk and some way, God only knows, missed the man. The clerk then drew a weapon and put the attacker down. Police investigated the shooter and found him to be a 14 year old male. The robber died from his short career at being bad. The sad part is the media stated the parent of the robber said it's the court systems fault for letting this boy get to this point. Might be some truth to that, but how ironic for a parent to even remark on such a tragic end to the son they tried to raise???
Anyhow I have went on and off topic but the point is, in the areas of Michigan I live and work it's very unlikely an officer be stationed at a library. I have heard of a security officer working at a few of the bigger city ones though.