You will get the dart frog after you rescued the wizard from the grim mountains. You will receive the tranquility spell from the Dracula book after you have flicked through around 360 pages.

They are not secret doors. Sometimes they open up and the ones you passed through then closes when you are in a different level. The combination of open and closed doors give the dungeon an endless feel to it. Observe the positions of the closed and open doors when you are down there. I noticed it when I entered a door then went down the stairs to the right and then there are swinging hammers on both tunnels. It repeats several times, but in some levels both of them are open, in levels only one is open.