How do mentalists work ... I just watched a mentalist show ... he knew what people would say what word they chose from a book and what object they were holding while blindfolded. How do they do it?

All I can tell you is that there is a way that the mentalist can make the spectator choose what it is he wants them to choose. In the case of the book he used a book force, did he use a deck of cards, because using a deck of cards to determine a page is a frequently used method. Even if he didn't use a deck of cards he would have forced the spectator in some way.For knowing what an item is while blindfolded, it is most probable that he'd have been using a mentalists blindfold, which is essentially a fake blindfold. He can see through it. Quality mentalists blindfold can be tested by a spectator and it appears to them as a regular blindfold but the mentalist will use the blindfold in a special way which allows him to see through it.I use these effects all the time and it's a lot of fun when people have absolutely no idea how the hell you just did that. There are many other methods to gain personal information about people you have never met before and this all falls under the realm of mentalism. Hope this helps.