When you are still learning, it is permissible.
But when you've already memorized all the necessary recitations (al fatihah, and short praises and supplications in every movements), it is better not to read from book, because in shalah we must concentrate as if we are standing in front of Allah SWT.
In one of the masjids that I go to, there's a very large Al Qur'an placed next to the imam, just in case the imam suddenly goes blank on a surah.