Grandmaster Nigel Davies (information about him can be found at lichess) has written some fine chess books and created some educational chess videos.
His books have looked at chess improvement, for example: Rules of Winning Chess, The Chess Player's Battle Manual: Equip Yourself for Competitive Play, and 10 Great Ways to Get Better At Chess.
GM Davies has also examined a number of chess openings.
My favorites of his books are Gambiteer I and Gambiteer II.
The Death of Certain Openings
The dawn of high power computers has meant that opening theory has been revolutionized. One of the main effects is that some openings that were previously thought to be playable are now in deep trouble.
One of the highest profile victims is the Benko Gambit (1.d4 Nf6 2.c4 c5 3.d5 b5), in which Black may no longer get compensation for his pawn. It goes without saying that gambits such as the Latvian (1.e4 e5 2.Nf3 f5) and Elephant (1.e4 e5 2.Nf3 d5) are doomed, but it seems that even the venerable King's Indian (1.d4 Nf6 2.c4 g6 3.Nc3 Bg7 4.e4) is in some trouble. Top players have been moving away from it in their longer time limit games, though it is played quite frequently in rapid chess and blitz.
It goes without saying that humans may not be able to learn and remember the engine refutations, but playing a dubious opening can eat away at your self-confidence. What if one of your opponents has actually learned and remembered a highly effective line? Will you be able to overturn things in a practical game? And is it worth investing time and effort in finding new resources that keep it alive?
Personally speaking, I would prefer to play openings which are still considered sound, but offer chances to outplay an opponent. There are plenty of lines left which fit the bill, for example most of the Queen's Gambits (Accepted, Declined, Slav etc) together with the Nimzo-Indian (1.d4 Nf6 2.c4 e6 3.Nc3 Bb4) and Gruenfeld (1.d4 Nf6 2.c4 g6 3.Nc3 d5). Against 1.e4 there seems to be little immediate danger to 1...e5, the French, the Caro-Kann and most lines of the Sicilian.
How can you tell which lines are still healthy? A good indicator can be the games of top over-the-board and correspondence players, if they're still being played then probably they're in decent shape. If not then beware, many openings are dying because of high-powered engine analysis.
