3: Broken Valley, Secret Passage, a book on the floor beyond the locked door Martis opens, past a spider web.
This one I'm a little confused. I have DKS xbox version. Never found a book on the floor (I could have missed it) but there is one in a chest in a room that opens when you press a hidden wall panel after freeing Martis. Sometimes this room is not accessible because of a cave-in (I don't know what activates the cave-in).
I think that they're talking about the same thing, it might just be a mistake in the FAQ. That room is accessible after the cave in, by pressing a secret button behind a spider web. The book is in a chest, and you found it.
The room is only accessible if you freed Martis, as he unlocks the door. OR if you mind-read him for the password in jail and then entered the cave anyway, the door starts unlocked, and there is no cave-in that happens.
The cave-in is triggered by releasing Martis from his cell without mind-reading him.
There is one optional room earlier that has a trapped treasure chest that locks you in a small room with two skeletons, but there's no skill book there as far as I know. That one is blocked by the cave-in. For some reason, you can't re-enter the tunnel from the chapel after the cave-in.
