Isn't one, actually. Nothing in the rules is clear-cut on the question of botches and rituals, it seems.
If there is no stress, there are no botch dice. However, I might say that some Rituals will ~always~ be stressful, by their nature. That, it seems is not defined in the RAW.
(On page 81, the small paragraph in the middle, says ""The magnitude of the Ritual, and the need to incorporate many elements, mean that they are always cast using a stress die."
I would take that to mean - "always a potential to botch", but that's just my reading. I don't see any other "stress" that would usually arise in a ritual, other than the innate problems associated with such a lengthy process. ymmv.
A mastered spell reduces the botch dice ~if~ they exist.
Our troupe has also interpreted that rituals always use a stress die. (but usually only 1 or 2 botch die, since you are often at a familiar location etc.)
So the remark that a smart caster will master the Aegis of the Heart is correct. But the easiest way to get xp in mastery is repetetive casting -> quite expensive for Aegis of the Heart.
So I must conclude that a few books on Aegis mastery are written, but they are extensively copied.