"The Next Big Investment Boom" by Mark Shipman.
(How to profit from commodities).
In hindsight, it's probably not that great a book, but it's what first got me interested in trading, trends, and TA, so in a way, it's had the greatest influence, although I've learned a lot more elsewhere since.
Later on I became quite enamoured of the whole Turtle thing, e.g. "Trend Following" by Michael Covel, but again it's probably not exactly a great book.
More recently it would be "Currency Trading and Intermarket Analysis" by Ashraf Laidi, which doesn't really teach you how to trade, but does help give one a deeper appreciation of the markets.
There isn't a single book that I'd now recommend, but I would recommend reading widely. One can always learn something.