I remember the Landmark books, IIRC there was a U.S. series and a World series. I remember one about Alexander the Great, his horse Bucephalus, his friend Black Cleteus. In the U.S. series, I recall one on the Alamo, Custer, Ethan Allen and the Green Mountain Boys and the U.S. Marines. Another favorite series was called "Through Golden Windows", a mixture of history, historical fiction, poetry, humor, etc. Another favorite was entitled "They Put Out To Sea", about the various explorers, Magellan, etc. Also back in the 1950s I was bitten by the Civil War bug, I was fortunate to inherit sets of the Douglas Freeman books, Lee's Lieutenant, R. E. Lee plus the 10 volume set Photographic History of the Civil War.