O'Neil said that the best book on investing he had ever read was The Battle for Investment Survival by Gerald Loeb. In Loeb's book, there is an essay that advises one against divrsification (which O'Neil himself would reiterate in his own works) and instead advised "putting all your eggs in one basket, and watching the basket."
Nicolas Darvas, author of How I Made $2,000,000 in the Stock Market was also an early influence on WON, and he claimed that he read and re-read Loeb's book weekly.
You really don't need any other books to make a go of it in the stock market, in my opinion.
I'd add Lefevre's Reminiscences of a Stock Operator, Darvas's How I Made $2,000,000 in the Stock Market, and Loeb's The Battle for Investment Survival to the list.
Those four are really all one needs.
William O'Neil's How to Make Money in Stocks is really almost a textbook of Darvas's "techno-fundamentalist" theory and the lessons of the other two.