I'd say the rule book is more or less correct, on page 280, but in this case only if the character behind the prone character is also prone, and only if that character is close behind the one providing the cover.
This provision of cover comes at a cost (at least in my ruling) that about half of the miss percentage provided by the prone character providing cover means that that character is hit.
To explain: if I'm trying to shoot an arrow at a prone target, my vertical miss chance is obviously higher (thus the +2 to defense against ranged attacks). If I'm trying to shoot an arrow at a prone target hiding immediately behind a log, a given percentage (depending on the percentage of cover provided by the log) of my shots that would have hit my target will hit the log instead. In this case, the front prone character is the log.
I usually make it a spot ruling, based on the size of the front prone character and the distance to the ranged attacker, as the greater the distance, the lower incident angle of the attack's trajectory. As well, the larger the front prone character, the bigger the "log".