Frankly I never found the spy thing believable. Sure he's a very clever guy as well as having an engineers brain but he's also a "what you see is what you get" type IMO. That's a big part of his charm, he's genuine and wears his heart on his sleeve. Impersonating an entirely different race for months (years?) at a time? It just doesn't fit. Still, I read the ENT books anyway as there simply was not enough ENT on tv and they have been pretty good as far as Treklit goes. I enjoy having more ENT.
But gah, I hope they drop the spy stuff soon!!