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1) can anyone point out other outdated theories in the book (if it matters, my copy is the third edition from 1978)
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Most of what Magriel said is both valid and worth reading repeatedly.
Another category of errors which comes to mind is the action play section. If I recall correctly, most of the examples were wrong. The action play idea is ok at times, but it is sensitive to the tactical situation. Magriel applied it too much.
You can find discussions of particular positions in Bagai's book
Classics Revisited.
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2) what more recent books have been written which would be helpful in learning how to play backgammon better (or alternately, are there any really good forums online for backgammon, the few I have found didn't seem to have all that much traffic).
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The best online forum I know of is
www.gammonline.com .
There is a lof of good material in the
rec.games.backgammon archive. That has many discussions of books on backgammon that have been out for a few years. Note that Bill Robertie's books are good, and he is also a 2+2 author, and occasionally posts in this forum.