ok well i'm going to go through each of these now:
compass:
the needle is off. If you are intending for it to look 3D like it's raised in the middle, then we should still see the other side some and the bisecting line would still be off. I personally would keep it flat so the bisecting line would run between the "w" and the "e". Also, why so many strokes? The book has no strokes, the register has a few, and the compass has a bunch. I would lose them all. Make a bigger contrast in color between things and the stokes won't be necessary. and i think the Blue looks purple.
register:
This is my least favorite. Your drawing is good, but you need to reel it in some. #1 thing i would say is to lose the strokes around the keys and the pink thing. Also, pay attention to your lighting...everything is accurate except the pink thing. It has no lighting what so ever. ALso, on the blue part, it looks as if you've made your gradient in Illustrator with grayscale black to rgb blue. THat causes a lot of muddyness in the middle. Try rgb black to rgb blue, or if you really want to make it look sophisticated, use a darker blue instead. Also, there are no shadows from the keys (which is probably alright), but i really think there should be some subtle shadows to them. Also i think there should be fewer keys. As an icon it will look better if there are fewer on there.
book:
this is my favorite one. Ad some lighting to the book mark, and call it a day. (edit

the book is also a little purple.
hope that helped, and i would've written all this originally, but i was looking at it all on my phone and just typed from my gut at the time.
-JE