I'd push them to the wall and then use trim pieces to cover the connections, as well as the small gap you'll have on the right if you don't remove the baseboard. A bit of crown would be nice too.

http://centsationalgirl.com/2011/11/from-billys-to-built-ins/

Centered.

centred for sure

Another vote for centred.

push them to the wall, if not what you will do with a stupid small space?

Depends a bit on how close the wall to the left is -- if it's close, then I'd definitely push into the corner, to keep the doorway from feeling crowded. Otherwise, either is ok, depending on whether you have a vase or something else you'd want on the door side of the bookcase...

Can you buy another section like you have in the center or would that make it too wide? If you are leaving these shelves then I would center them for sure.

How much space is on the other side of the door?

None

Then I'd push it to the corner so you aren't over-crowding the door.