a rutter is a 'thing'! they were so precious that the enemy was not allowed to get them. they were rather thrown overboard than lost to the enemy.
i like the general idea, but can see it used for a whole new area of gameplay; merchants or nations trying to steal rutters from the competition to get the edge on them.
one should be able to steal a rutter from a fleet
and rutters should be coded. the book is not enough, a 'key' is needed.
research could improve the coding/decoding levels, but everybody should, by spending lots of time on it, be able to crack the code
auto trade fleets have now an admiral and a merchant. i think that this could be the place where the rutter could be stored. (only one space for one specific route). unused rutters are stored in the same place where unused ships float around

. a place that is somehow a tiny bit vulnerable for thievery.
(the Dutch and English banded together to steal the spanish rutter to the East.)
anyway, this is just a suggestion.