Last year I had a similar experience as I was leaving the largest (and probably the only "real") technical book store in Houston, Brown's Book Store. Bruce, I'm sure you've heard of it.
I went in to purchase around $500 worth of Houston building codes/Texas State fire alarm manuals and software.
Right as I was walking out the door I noticed a stack of non-technical books in a large metal basket with a small sign above which said, "Bargain Bin".
Being the big cheapskate, I wasn't about to leave the store without at least taking a peak.
What I found inside was a goldmine of old advertising books from the 50's and 60's. I grabbed up at least four or five "keepers" in the stack.
When I got to the counter, the woman was happy to charge me a whopping $12.00 for the entire lot. As I was leaving again, I heard her say to the assistant...
"finally, we got rid of some of the throw-away items".
They obviously didn't know how valuable these book were.
I don't know what the names of these books are offhand, but I'll see about taking some pics and putting them on the board along with my growing library.