Well, we all know that spending time in the company of achievement minded, practical people tends to help with achievement. After all, friendships are really just a regular series of communications with people we tend to agree with. We can even be changed / motivated by people we've never actually met, such as politicians, actors, preachers, whomever. So clearly you don't even need a two-way communication for this to work. It's also possible to be affected by an 'enemy' if you spend enough time with them. So clearly you don't even have to see the person as a friend to accept influence, although it's likely that the less you respected the source, the less influence they woudl have. And it's also clear that, unless this process takes place over time, it's unlikely to be 'life changing'.
I can see how a book of repetitive phrases would tap into the same process that causes us to be influenced by friends and other people we respect. The book is really just a way of a message being communicated back to us. You could probably think of it as a hack to get positive ideas burnt into your subconcious on a large scale. And also, it's obviously important that you respect the book (and by extension, the author). Friendships work when you let the friends have influence over your life because you value their opinion.
I read a lot of these types of books, and I've been questioned why. I figure I'm just going to spend the time either reading fiction books or watching TV, anyway, so why not?