This is all in all a western yaoi novella. Kail is the classical uke, all blushing and tears, all cuddle and petting. Master Nerrin is the classical caretaker Seme, the one that gently accompanies his pet to discover all the sexual potential he has in him, even when the innocent uke doesn't know what he really wants. Master Nerrin is gentle but firm, once he accepts Kail's plea to be still his pet, he will never allow to Kail to take back his decision.
At first Kail searches Master Nerrin's attentions since he wants back the feeling of love he had from Master Nerrin when he was a pet, but don't worry, there is no reference to "sexual" relationship when Kail was a dog: what Kail feels now is totally new for him, like Kail was a virgin at his first sexual experience. This is something that is not exactly clear: I don't know if Kail was so innocent even before he was turned in a pet, or if his actual behaviour is still a consequence of his unwillingness to be again a man: I believe the reason is this last one, since Kail behaves very much like someone who is facing for the first time a set of all new "human" emotions.
Anyway, if you like yaoi, and you like a nice and enjoyable long sex scene, The Pet Promise is a nice book; but mind you, if you don't like or know the Seme and uke's interactions, you probably will not understand this one, and Kail will result a very improbable character.