Summon Water Spirit (Aion Skill)  

It appears that the drop rate for these skill books are quite random. I had been playing Aion with a friend this past few days, with them starting a Spiritmaster first, and then with me starting my own a few days later.

One of the first water spirits that she killed at the shrine dropped the book, but we must have killed a few hundred before the book dropped for my character!

I also noticed that we weren't able to find the book on the wind spirits in the area, though we had a couple of quests to hunt them down. They seemed to be more than happy to drop the Wind Spirit I summoning book, as they did at least five times before we finished the first quest, but if they happen to also drop the Water Spirit I summoning book the odds seem to be a little less (as would only make sense).