i hardly ever use them, only if i cant do someting for a week and its annoying me badly...i try not to though, it ruins the game.
i remebr when i first played re, i got stuck on those doom book for a few weeks, i knew there was ssomething you had to do, but i didnt relise you had to open them!!!
i went round the whole game thinking i missed something, then a friend told me, but only that bit