Looking back these were tricky ones. Riddle four is "In the light of the morning sun, the only walls are in your mind." The sun rises in the east and the east (right) wall of the room can be walked through, leading to a hidden note with the code. Riddle eight is "If you're puzzled by a problem, stop and think for a minute." The solution is to literally wait a minute after reading the hint. As long as you don't move from the spot in that time, you'll get a message that a code magically appeared on the page, and examining the same bookshelf will show it.