"History of Art for Young People", by H.W. Janson & S. Cauman, second edition revised by A. F. Janson, 1982. Published by Abrams. Almost 400 pages long.
It's at adult reading/sophistication level (in my opinion)-it was given to me in my childhood but I didn't bother with it until I was grown up. It's not "childish"-it's dense, intellectual, and has many illustrations. Maybe there's newer version, but this is the one I have & it's pretty comprehensive (of course there's nothing more recent/contemporary than the 1970's).
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*"I don't know what it is, but I know what it isn't."*