Very nice bookcase. I like the contrast between oak and walnut and use those two together often.
I don't know how wide it is, looks about a meter? If so, you might want to reinforce the shelves to prevent sagging - depending upon what weight they will see. I made a bookcase from oak a while back, about the same size, and used supports on the front and back of each shelf.....see attached photo. These were a couple of extras left over, worst of the lot, now in the scrap bin. The left one goes on the front, obviously, that leaves the right one for the back. The shelves sit of the rabbet and add strength to the shelves. As can be seen, the left one that goes on the front is more decorative, and these are somewhat simple to make on a router table with a couple of bits. Just something to consider....