The rule book states that reserve cards are placed face up when reserving. But there is a Strategy Card that lets you pay to view your opponent's reserve cards? So is the proper way to reserve cards face up one at a time so as to only let the top card be seen?
It's normally public, but there's an optional rule in the Companion Guide that lets you play with them hidden. If you aren't playing with that optional rule then you can ignore that bit of text on the Spying card.
This is also covered in the "Errata and Clarifications" post.