Once a toddler learns to read a custom made book, it is not a big leap for them to learn to read a published book. The main difference being that the custom made book uses words that you know that they know. If they have learned to read phonetically, when they read a published book, their vocabulary will naturally and easily expand. This is the time to teach them new phonetic rules that they may not have learned earlier, for example that the k in knee is silent. But once they are reading with confidence, this will be quickly and easily learned.