I once flew for an airline that did not allows books at the exit row. The reasoning behind this was that if a situation was to occur (eg rejected take-off) the passenger would not be able to keep hold of the book. The book it self would become a projectile.
As for mp3s on landing, while I can't speak for all crews and airlines, it sounds to me like the crew have simply missed you with your mp3. At every airline I've flown for the same standards are required for landing AND take-off. Funny enough, at the airline I mentioned above, a senior cabin crew member would follow the cabin crew to double check their cabin secures. Any discrepancies would be mentioned in the de-briefing.