Fascinating reading about your enormous collection! I have collected books since childhood and also inherited a few from family. I homeschooled my son and with the addition of those books, I guessed I had about 5000. When my husband died and my son was in his 30's I went through and got rid of many of the books bought for homeschooling. Almost all of my books were bought at tag sales or book sales. I seldom paid more than 50 cents for a book, or 25 cents for a paperback.
I don't know how many books I have, though they are all cataloged in notebooks that I used to take with me to the tag sales, etc. I also have a collection of National Geographics that go back to 1914 that were also collected from tag sales. My son liked them when he was small. It's not complete from the 1950's back, but after that it is.
Loved this post!