...for any part of the game (shot making, shape play, mental aspects, gambling...), and any source (books, DVDs, other players, forums...)?
The best advice I've ever received is to not give out the best advice I received to anyone else!
You just did.
If you gamble , post the money. If I had a dollar for every person I saw stiff someone or who bet money they never had and busted someone, I would be able to buy a nice Zsamboti.