The Lessons were written by Vivec...who is the Temple's god! To suggest that it isn't considered a holy book is blasphemous.
I didn't say that it wasn't
considered a holy book, only that it isn't one, and even those who
consider it as one don't consider it as a central one, and that those people aren't likely large in number.
Although, given the nature of the Lessons, not to mention its title, I would certainly think that they were meant to teach something and would thus be considered a holy text.
They are meant to teach something, just not to the religious followers of the Temple.