[BanazirGalbasi]

The idea is that there's several layers. If the spy appears like a normal citizen, then they won't be surveilled closely, so the book is less likely to be discovered. Keeping multiple books around means it's less suspicious that they have any given book - you just like to read. Why would the specific book be "discovered" when it's in a stack or shelf of them?

You also don't have to send an encoded message in every letter. If you have plenty of plaintext correspondence with a distant friend, and only some of it has any special meaning, then it will be less suspicious in the first place.

The whole idea is to blend in and not attract surveillance in the first place. Your counter-argument is based on the premise that blending in has already failed, in which case you're right - the game is up and the spy is compromised. Worse, it's possible to intentionally feed them bad information without them knowing (which happened multiple times in WWII). But this entire article (and the book it discusses) is about preventing that entirely.