I've yet to meet a straight wall or perfectly square corner.

Usually, when a built in is made, the face frame along the wall is scribed and trimmed so that it's a tight fit. You don't have a face frame, but you could add a trim strip / molding and scribe and fit it. This is done with kitchen cabinets all the time.

As for the back panels, are they part of the wall or part of the bookcase? There's no reason why there should be a gap if they are part of the bookcase. But with the electric outlet there, I'm guessing they are part of the wall. Again, perhaps a trim strip or quarter round molding. The positive side is that if the bookshelf is filled with books, you won't see it. If it's for display items like pictures and knick-knacks, well, something should be done.

Thanks so much - I'm relieved that there is a relatively easy fix for this! The panels were added onto the wall behind the bookcase, and then the bookcases were assembled onto the wall, if that makes sense.

Most of the reason for trim in the first place is to hide gaps that are normal. Caulk is a narrow gap solution, so trim needs to be installed first. I use caulk to hide the tiny lines where trim meets the other parts(for painted woodwork) for a really slick look.

One way for the left side of that bookcase would be to rip a thin strip of wood to fit in the gap and then caulk to hide the lines. Quarter round or shoe molding would work for the insides(or made on site triangular molding).

Awesome, thank you HandyMac!

I totally agree about scribing a face frame but telling the contractor and him having the skill is two different things. I only say this because A. Pros would have done it from the begaining and B. The contractors comment when asked about it lead's me to believe he's not skilled. What I'm getting at is he might need to sub the additional work out.

There is no face to scribe. A flat molding should work on the sides.

Taylor's C&I,so that we can all keep up with the latest and greatest, can you please elaborate on the difference between face frame,trim and molding in this instance. It might also be interesting to hear why addition of plain 1x2 isn't suitable to this casework. Thanks.

What are you planning on placing on the shelves? You may want to consult a shelf strength calculator (sagulator) before loading them.

Good idea, books can get very heavy quick. Weak shelves can be strengthened with molding applied to the long edges and/or doubling the thickness of the shelves.