This book is kind of hard to classify. Each chapter is a description of a particular "lost city" of Equestria, with no plot or characters per se. But the description is prosey and describes the historical events surrounding the city's rise and eventual fall, making a story out of it.
It's really an exercise in worldbuilding. Quite well done, too. This might actually give some good ideas that could be reused in a roleplaying game setting or when writing some other fantasy story, and the reliance on the My Little Pony setting is modest enough that it's not strictly necessary.
So I recommend this book for game masters, people who like fictional histories and worldbuilding, and that sort of thing.