I agree Muay Thai Unleashed is good.Also Ultimate Muay Thai by Bob Spour.

There is also Muay Thai The Most Distinguished Art Of Fighting by Panya Kraitus,which is considered one of the definitive books.Although it has virtualy nothing on clinching ,which is strange.I got mine from Thailand so I dont how easy it is to get.
I point my saxaphone at the rare Booted Gorilla.