Built in cabinet spacing
That looks perfectly fine.   

Two notes:


I would put a block on the back and the front. Since the cabinets are not screwed to each other you need to join them. This is as simple is a 6" inch piece screwed to wall then maybe one screw from each cabinet into it. On the front I would try to get two screws that are a few inches apart for each cabinet (would look for scrap 2x8).
I am assuming you will have a surface for your bookcase? The gap shouldn't really affect it either way as you should be getting some screws into those two walls (given your bookcase is in one piece, not two small bookcases).


Aesthetics-wise looks normal. Probably wouldn't even think about it if there were doors on it but unless you had these things custom made that look is about as good as it gets. I think you would be the only person that even realizes there is a gap (you can make this a functional hiding spot too).