Some of the most interesting books I've got have come from clearing out friends or rellies houses after the previous occupants have moved out and left old stuff behind.
"The Electric Motorman's Manual", 1898;
"Chambers' Information For The People", Vols 1 and 2 (latest date I could find mentioned in there - 1865);
"Ripley's Believe It or Not", 1929;
some 1930's "Saturday Evening Post" magazines;
A variety of local newspapers dating back to the 40's under lino floor coverings or in cupboards;
plus a 1920's battery radio receiver, minus valves;
It's always fun exploring other peoples' interests...