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On Chess.com, there are Endgame and Strategy sections in Chess Mentor Lessons and Videos.
Amazon.es stocks books like Ivan Sokolov's Winning Chess Middlegames, Konstantin Sakaev's Complete Manual of Positional Chess and Mark Dvoretsky's Endgame Manual. New in Chess is the publisher of the first two books, so their website probably has these books as well. Forward Chess sells ebook versions of all three.
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