I would square it off with the fireplace/built in - add doors to the unit to mask the angle. You will still get storage but it will look intentional to the current room.

One thing - as we thought through this option and the unit would end in the middle of the wall? So you would have a triangle basically? In the end we would not be able to have much book shelf space and I think the wall's angles stand out. We would have a blank wall for art and a less useful bookshelf. This is what I didnt do so far, but I want to hear other opinions. Thank you for the thoughts.

Here is the plan if you straighten out the bookcase area. The angled wall offers an alcove for reading. Why is the seating arrangement not facing the fireplace and TV?

Unfortunately the 3d is based on what we expected from the blueprints. In the field, we had to change columns as we were finishing the basement and/or we also found a beam. There is a beam in the wall and so the area cannot be reshaped as you have it. That is the issue - we can't make that long angle you made because the wall behind it has a beam which is not where they planned (or they did not foresee this issue and didnt want to sacrifice closet, but not too late). So the area has been built and sheetrocked and it looks like what you see in the last 4 pictures (similar in shape to the 3d but a slight shift - but we can't straighten it out as you have. If we do, it ends up with a cabinet which ends in the middle of the wall. You can actually see they marked it in the picture (last 3 show it on link below):

https://photos.google.com/share/AF1QipOCYHsw2rO77YRM9HxLXjF34hMB2Hz8ScqAB5aSx8JcHrvaG7XtUyrFanj6Usq6qQ?key=M2ZVR1htREZLWWg4Tmx5UVVPdTZnSUNKZWE2UWVR

the left side of sectional which has the chaise is in front of the tv so we can watch from there. the window seat is also there and the arm chair can be moved. I think a bookcase would create another area of the room I guess.

here is the original layout of the room - you can see on right. we only have one armchair on left and another on right (versus what is shown here, you can see in picture). You can also see we used to have a small alcove room where the wall is closed off. That alcove became our front entrance.

Took some pictures. The angle changed from original plans and so the version of cabinets which square with fireplace becomes very short.

Current conditions

What if you tried something like this to balance the angled wall on the other side of the room instead of trying to square things on each side of the fireplace:

So I was just reminded by my architect as I contacted her again. We originally were going to have space for a 48” cabinet but then lost a foot after the wall changed. this is similar to your proposal which is squaring away the cabinet to match the other side.

the alternate idea we have the new shop drawings for, accept the angle and try to use the full length.

the version shown by palimpset was the original idea - and i put a drawing from my arch above. however it used to be 60" and now is 48" so it falls in the middle of the wall and is not much in terms of storage. it just ends kind of weird. There are ways to have a blank area if i do the larger cabinet. I'm more inclined to have a bigger cabinet, so trying to find best ways to do that if possible. here is actual today and there is an hvac element behind the wainscotting on the left so we cannot move that wall either.:

This is @palimpsest version plopped on your archi's drawing, just adding a wall to mirror the other side of room. If you are dead set on getting another 12" of cabinet storage beyond this 48", I don't have any ideas that still look like a built-in.

i will try to correct that arch picture. the wall on right in picture is only about 36" long now. so it doesn't give us the space we would like for the ideal scenario. the picture there would have been perfect but it isn't what we have - do you see my image above with pictures of current situation? i pasted the current situation now. but more importantly there is an hvac element on the left wall - please see the other picture shown below.

I've seen all the pics and architectural drawings, maybe I'm missing something. The intent is to match the wall on the opposite side of the Living Room doorway, which appears on the drawings (no pics of that) to be an angled wall (created from the juxtaposed Vestibule to exterior). Doesn't create more storage, but prevents the cabinet from just "dying" halfway into the wall, eliminates more shadows, and may help with the coffer/beam layout on the ceiling.

Here is the room before construction:

the area I am working on now is on the left side of the fireplace you see pictured here. There is now a wall there now and so we are trying to put a built-in there. It doesn't need to match the right but it is better if it complements and considers the right. It is meant to help us store books as we have a lot and no other real bookcase in a pretty large house. your original solution is nice but the wall as built doesn't quite angle as the image you have. That was our original plan but a beam/hvac location has changed possibilities. So the wall on left which angles and has the exit out of the room eventually at the end is not able to achieve what we expected. So i'm trying to get ideas on the best type of bookcase to put on the wall on the left. I think it should complement but doesn't need to mimick the right. We have some unused portions as you noted as well. Here is the shape of the room in full though the angles on left of fireplace are slightly different as built.

I do not care about matching the angle on the other side of the doorway. That is not necessary. I just want to make a bookshelf which is useful and attractive in the room and considers the oddities. At this late stage in the construction and renovation which started in april. we will not be moving any more walls. the new wainscotting and soffit are in place and I think we need to work with the angle we have but make the best built-in there.