Pharmacotheon, definitely. And I'd highly recommend Voogelbreinder's Garden of Eden as well (if there are any copies left). Between Pharmacotheon and Garden of Eden, I'm really torn as to which I'd recommend first... Pharmacotheon is a pleasure to read, and contains thorough information on a lot of the more common "entheogens," while Garden of Eden is written more in the style of an encyclopedia, but contains more information on all sorts of obscure plants. But I'd definitely pick either one over Rätsch's Encyclopedia.
The Encyclopedia of Psychoactive Plants looks nice... plenty of pretty pictures. But it's really not a very good source of information. Rätsch is sloppy with citations and includes a lot of hearsay (without necessarily indicating that it's hearsay). Not a book I'd recommend spending the money on if money is tight (though if money is not an issue, go for it... I'll admit I do use my copy, even though I feel compelled to scrupulously check the information presented in it against other sources). It has perhaps one fifth of the information that you'll find in Garden of Eden, which is far more encyclopedic even if it doesn't have color photos.
