Since the previous recipe contained a fair amount of hazelnuts and originated the hazelnut praline in this recipe, I decided it would be reasonable to do this one immediately after to decrease making small batches of niche ingredients.
The Paris-Brest is a circular choux ring that is covered in almonds and filled with praline cream of some sort. The pastry was commissioned to commemorate the Paris-Brest-Paris bicycle race, which is a long-distance race between the two cities and back, and the circular shape of the pastry is meant to evoke a bicycle wheel. Variations in the filling are common, ranging from diplomat cream (stabilized whipped cream) to pure buttercream and everything in between, though the praline (caramelized hazelnut) is a typical base flavor. The version in The Book uses a mousseline cream, which is what you call buttercream + pastry cream + a flavoring (in this case, praline).