Please post the photo. Most don't want to click on a link.

Thanks for your suggestion. I added the photo in the tool and re-published the post, however I can't see it. Not sure what is happening there.

Post it in the comments

From your description it sounds like a sheet of drywall and a bucket of mud would improve the situation.

Here is the photo he didn't want to post.

Check the opening for square. Then build a case that has an attached back & that will slip into the niche. Before fitting it, attach a face frame with extended stiles on the sides & top of the carcass.

The extended stiles will cover the gap between the case & niche. You can lightly fasten the case to the niche framing with finish nails or long trim screws, 2 or 3 per side. Beef up the shelves if you plan to load it with books to limit the amount of sag.

Great spot for a secret hiding place. Build a shelving unit with an accessible back.

I agree drywall and a bucket of mud those are not a good place for any kind of book case IMO those niches do not make sense there at all.

What might make a fitted bookcase more aesthetic would be to close off part of the top niche. Add framing to make the top niche finish out to the same width as the bottom niche. Then fit open bookshelves on top and drawers on the bottom. Drawers on the bottom would utilize the full depth of the bottom niche. Or full depth shelves on the bottom with doors. Preferably drawers though.