How can I get back the magic book or magic orb? Seems it's very easy to get this disarmed / taken from you - can't find it anywhere on level, doesn't turn up with your other lost items in the chest after you complete the level...

If you get disarmed by an enemy in a fight, the weapon drops on the ground somewhere close to that spot, usually within a 5 or so tile radius, but it can be around corners and such. If you get disarmed through a dialouge the item is lost. The magic book is a very common item to encounter and the orb isn't terribly rare, either, so you can replace your missing items with relative ease even if you can't find the originals.