The federal court ruling declaring that secular humanism is a religion is one of the most grievous assaults on First Amendment rights in this country's history. This shocking ruling is ludicrous on the face of it, since "secular," by definition, is the opposite of "religious."
Religious humanism does exist. It is embodied in organizations such as the American Humanist Assn. and the American Ethical Union. These groups espouse atheistic/agnostic humanist beliefs and have achieved the legal status of religions; their property is tax-free, their leaders may perform marriages, etc. Organizations such as these represent the religious expression of humanism.
Secular humanism on the other hand, if it exists in any coherent sense at all outside the imagination of religious fundamentalists, is simply an expression of mainstream American cultural attitudes. It has no formal organization behind it.
If a textbook writer wants to know what the religious beliefs and dogmas are of secular humanism, in order to avoid expressing them in a textbook, there is literally nowhere to turn. There is no published dogma, no priests to rule on what is and is not secular humanism.