I don't think there is anything essential written in Raanaar runes, which would require having read all three books. What riddle are you having trouble with?
The second Raanaar book is in the room just after killing the shaman, which you should have access to until you touch the crystal fragment. The third book is easy to miss, since finding the wisdom symbol will teleport you out of that level right away, possibly before you have finished exploring.
I find walkthoughs in general best used when you are stuck, or to skim though after you complete a section of the game, to make sure you didn't miss anything.
