A good book about FOREX

Recently a friend asked me about any book related to currencies. He just wanted to get the flavour of the Foreign Exchange world to understand his multicurrency mortgage. There are several good books. However, this one, Bird Watching in Lion Country is particulary easy to understand and it covers all the stuff: from different techniques to calculate the fair value of a pair or cross, such as Purchase Power Parity or Big Mac Index,  to technical analysis.