The books by Cornelius Luca seem quite informative. I've read "Trading in the Global Currency Markets", which I found good as an introductory book for the FX markets and trading.
Couldn't find really good forex books for some time.
Only until recently I found Callum Henderson's Currency Strategy - The practitioner's guide to currency investing, hedging and forecasting (Wiley Finance, 2002).
Targeting three types of readers: Corporation, Investor, Speculator, he introduces a Signal Grid framework to determine Buy/Sell decisions by integrating four common approaches: Currency economics, Flow analysis, Technical analysis, and Long-term valuation (i.e. FA).
Personally I think this is a great one in its domain.