My opinion is that it's a GM book. You can learn about almost every Free Captain, their alliances, and their motives in that book (Bedu Hanji is a WHAT??), as well as what islands to avoid until getting to higher levels. It doesn't have much in the way of spoilers specifically for Skull and Shackles, but if you have the book, I'm sure you were planning on using it, or at least having it handy in case you needed it, in which case, letting the players read it wouldn't be much different than letting them read part of the AP.
If they've become very familiar with a specific island or settlement, you could choose to let them read information on just those places, but giving them free reign to that book doesn't seem smart.