Carpentry and Woodworking - Assistance in building a bookcase

I am building matching bookcases with a "desk" between them. My problem: I live in an OLD house and nothing is square or level. The first bookcase slopes about 3/4" at the ceiling level and 1" from bottom to top making it look very unlevel to the room. (I am hoping this will be disguised as the bookcase is filled, curtains added, etc.) When I build the second bookcase, should I continue to build it level (making it appear larger than the first bookcase) or should I creatively shorten it to look level. (Or am I way off and I should start over?) This one has me stumped.

After installing a number of such things in old historic houses (or should I say "olde" historic houses?) in downtown Charleston SC, I'm with you on the confusion. Our bookcases are already built to size in our shop when they show up so we've never considered altering the size of them. If it works for you, go for it. You can install crown moulding around the top of it to try to disguise the slope, altering the point of attachment on the bookshelf to more meet the cieling. The toss in some properly colored caulk, and you may have something going there.

As far as the problem with the vertical slope (assuming you mean the wall) you can fabricate a hardwood or plywood scribe to fit in between the bookshelf and the wall that about an inch long at one end, and tapered just as the wall slopes to a point. Ie, put the bookshelf in plumb, and use the wood cut to shape to fit in the long triangular gap. We do this with 3/4 thick, inch wide material like pine or poplar and use a chalk line to pop a line down the length of the scribe and freehand the cut on the tablesaw, which is a supremely dangerous be we have all our fingers. You can also make that cut more safely on a bandsaw and use a block w/rough sandpaper to sand it smooth.

We always show such problems to the homeowner before we fiddle with oddball installation tactics, and give them the option of level and plumb cabinetry that LOOKS unlevel and out of plumb, or to have unlevel, unplum cabinets that look kinda right, along with a healthy reminder that it's the house causing the problem and not the product causing it. They always go with the "make it look right" theory.

Sounds like you have some experience in the matter. It gets better. To avoid cutting into the crown molding that took days to install and get somewhat right (considering what I was dealing with and the magic of caulk), I left the first cabinet about three inches shy of the ceiling. If I make the second cabinet level to the first, it's going to look taller. Should I build it "visually level" to the first cabinet at its tallest point and then level the top and bottom fixed shelf from there? Thanks again!

I would also consider shimming the bookcase(s) at the floor to make them level and plumb. You could cover the shimmed base with some quarter round or cove.
If you cut-to-fit to make them right in the room that you're putting them in, then if you want to mve them to another room (or even another house) you'll have the same problems all over again