An angel and his apprentice seek answers from a magical book in post-revolution America invaded by angels. Epic quest ensues.
The story started with an statement of the time frame, which (according to my poor grasp of history at the time) should have been just after the American Revolution. Sometime around that time, angels had come to earth and made themselves known. They ended up gaining some power and made some sort of colony in the United States, though exactly where I don't remember. The angels were human-shaped and sized, with similar anatomy, but had no wings as a whole, though I seem to remember several scenes with various angelic characters taking flight.
The story followed a young boy apprenticed to one angel as if it were a trade he could learn. His parents were honored that he was chosen by the angels to be an apprentice through a conversation the boy listened in on. He wasn't excited about it at first and gave the impression that most of the angels were arrogant and unapproachable, but found his master was different. The angel treated the boy kindly and fairly and did his best to teach him about the world and about angels as best as the angel knew. The angel used magic on a fairly regular basis, but the specifics of it I don't remember. It seemed like he was a scholar of some type by trade but I don't remember it being specifically mentioned.
Together they were seeking some magical book that had answers to the questions the angel had, the nature of the questions I don't remember. Their travels were opposed by other angels who didn't want them to find the book, because the questions he was asking would change things somehow in a way most angels didn't like.
I can explicitly remember a scene where the two are escaping on a boat and are being chased by a few angels. At least one of the chasing angels had some strange powers, and was able to turn into a large group of insects, like ants or wasps, but the protagonists were able to get away. One of the angels in their group was lion-like with a muzzle and mane but a humanoid form, or had the ability to turn into a lion, I don't remember. It may have been the same angel and I've mis-remembered and combined the two. There was also a female in the group chasing them. I believe this group was the main set of antagonists, but were being directed by another party, another angel who had some sort of political clout.
Near the end the two had gotten to some large magical angel city and recovered the book, which gave the boy some magical powers like the angels. I remember his powers being described as blue flames, and the book may have given off blue flame as well. The boy ended up using his new powers against their antagonists and everyone was surprised at how much power the boy had and he nearly destroyed the opposing angels in a fight in the city. The book answered the questions the angel was asking and revealed they weren't angels at all, but elves who had to escape from their magical realm due to some evil destruction. This was a big surprise to the protagonists, though I explicitly remember being un-surprised, as the magic they'd described as having used seemed more nature-based than what you'd expect angels to use.
The book came to a proper closure with the good guys beating the bad guy and getting their questions answered, but it seemed like it would have been an opening for a series, with the boy getting new untapped powers and the angel/elf knowing the truth and having the will to try to inform others. I can remember looking through the book for hints of a series, any of those advertising blurbs paperbacks will occasionally do like 'here's a sneak-peek of the next book!' or 'Other books by this author!' and I don't remember being able to find any.
The tone of the story was serious with fictional elements. There weren't any scenes focused toward comedy, but there may have been some comedic moments I don't remember.
Brief backstory about the book: I found it as a softcover in a truck stop gas station while traveling between the Midwest and East Coast sometime between 2000 and 2002. I found it in a spinning softcover display among other cheap softcovers, mostly romance novels. I wasn't interested in romance novels, but this book caught my eye. The cover had to have been visually stunning enough to catch my attention, but considering the competition it didn't need to stand out much. I don't remember what the cover looked like, but may recognize it if I see it again. It seemed new at the time, not a reprinting, and was not by an author or publishing house I recognized.
I read it multiple times and enjoyed it greatly during my trip, but sometime near the end of the trip misplaced it. I revisited the same truck stop on my way back and was unable to locate another copy.
It may have been directed toward the young adult group, since I don't remember anything explicit - language, sexuality, or violence - about it. My memories are vague now and I might be wrong.