How many Books

My 8 year old asked out of the blue, the way kids do, "Dad, how many books are there?" 


After some discussion, it emerged that he was looking for a count of all the books ever published. 


Using rough back of the envelope techniques I teach engineering freshmen, I conservatively estimated an upper-bound of about 500 million. 


Then I looked it up online. Turns out Google did a count just a couple of years ago, and found that there were about 174 million known unique works.