How to build a recessed bookshelf?

My house has a wall that is already recessed… Sort of. It is more just a wall that has the middle 6 feet or so cut into a nook. I am wanting to turn this into a recessed bookshelf and need some advice/online links to look at. Also, any creative ideas would be appreciated.
I have found a lot of links on how to make a recessed bookshelf, but I haven't been able to find anything about how to do it if the wall is already recessed. I don't have much DIY experience and was hoping for something fairly simply, even if it just involves buying a bookshelf, sticking it in the cubby hole, and then putting some trim around it. I just want it fill the whole space and look good.
I included a picture. The recess goes from floor to ceiling with trim/crown molding. Ignore the radon test kit. It is about 1' deep.

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