What is the ruling on reading the book "Fervent Hearts" by Fethullah Gulen, and is it permissible to habitually recite post-prayer dhikr that has not been reported in the Sunnah?

It is not permissible to consistently recite dhikr (remembrance of Allah) that is restricted to a specific circumstance—such as after the obligatory prayers—and to adopt it as a regular Sunnah, if there is no specific evidence to support it. For acts of worship are based on divine command and adherence (to tradition), not on whim and innovation. Prophetic supplications (du'a) and remembrances (adhkar) are the best for a Muslim to seek out. No one has the right to legislate for people a type of dhikr or du'a that is not legislated (by the Prophet) and make it a regular act of worship; rather, this is an innovation in religion for which Allah has not given permission. As for what a person supplicates with occasionally, without making it a Sunnah for people, there is no harm in it, provided it does not contain a forbidden meaning. The prescribed adhkar fulfill all sound objectives, and only an ignorant, negligent, or transgressing person would deviate from them to others.