As soon as children can understand what you are telling them, you can tell them what the books would. An earlier question on here about kisses on booboos had this answer:
When my daughter was a preschooler her body reacted a lot to things others don't notice. One bug bite would swell her hand so she could barely use it. She was allergic to Solarcaine. I would tell her "tell your hand to stop swelling now" and she, not knowing that was a ridiculous request, would comply - and the swelling would go down.
(Why do kisses stop a child from hurting?)
So this approach worked at that age for the girl, even though she probably couldn't read and understand books like that. Start talking to your child about the power of your mind right away, and when they are old enough to read, it will be easier to understand those books, as they will have already been living those words.