The Prisoner of Heaven is a good sequel to The Shadow of the Wind and The Angel's Game. Prisoner is the shortest book so far in the loosely connected quartet, and gives quite a bit of background on Fermin. It is very much a sequel book, tying the two previous books together into a larger narrative, while also setting up the story for the next book. The main drawback is that the book cannot stand on it's own. You wouldn't read this without reading either Shadow or Angel's Game.