Things are starting to fall in place a bit. The shelves will not be a series of
case pieces, but rather a series of vertical ‘sides’ permanently installed to
support adjustable shelves. The sides will extend from floor to ceiling. Have
you considered making the bottom row of shelves fixed?  This would hold the
sides square to the wall, and eliminate the need to attach the sides to the
floor.

One other thing that you may or may not have considered ( please pardon the
unsolicited advise if it is not needed): it is virtually guaranteed that the
floor and ceiling are neither flat nor level. To get a good set of level
shelves, you can’t count on either as a reference plane.     So the first order
of business is to mark a level line around the room at a convenient height for
measuring to the floor.  This line should correspond with a measured height on
the sides, such as the bottom of a shelf support. This becomes the reference
plane for installing the sides.  You can either make the sides a bit long on the
bottom and then scribe them to the floor, or make them so that the side at the
high spot of the floor is sitting directly on the floor, and then shim up the
others to the appropriate height. At the ceiling, the sides will all have to be
scribed to fit. Of course, if you stop the sides short of the ceiling and
install a fixed top shelf, you can save some installation labor since you won’t
have to individually scribe each side to the ceiling.

A laser works well for marking the level lines, but if you prefer something
closer to galootish, we used to use water levels and chalk lines back before
lasers were cheap and commonplace. A water level is just a flexible plastic tube
mostly filled with water and food coloring.  Since the water will seek its own
level, you can use this to transfer a mark at a given elevation from one
location to another. It’s a 2 person operation. One person stands at the
location to be marked and holds the tubing against the wall with the open end
pointing up. The other person stands at the reference mark with the other end of
the tube against the wall, raising or lowering it until the top of the water
aligns with the mark.  The. The first person can mark the wall at the top of the
water where it will be level with the first mark. Then a chalk line is snapped
between the two marks.

To be honest, an accurate 4’ level is also sufficient to draw the level lines to
the degree of accuracy needed.  But I am reaching the age where I find myself
saying, “ this is how we used to do it back in the old days…”.

David