Having the book is one thing... Jobst Brandt's genius makes the mathematical facts very clear if you read it carefully. Here's my book report:
You wouldn't affect tension by weight-loading a completed wheel, except specifically at and near the very bottom of the wheel, where the earth's surface pushes upwardly through the [pretensioned] spokes to hold up the hub on a 'virtual pillar'. The book includes a reference to a proof that no other static forces arise (other than what would normally arise within an equivalent rigid body performing the same function) in radially, at the axle, weight-loading a completed (fully tensioned) wheel. The spoke(s) at the bottom (or other point of radial force loading) area will experience a net/total tension reduction similar to (there's some trigonometry involved) the magnitude of force acting downwardly (in the static scenario) through the axle (which includes most of the wheel's weight). The profile of spoke-wise distribution of tension reduction will be a complex function of tire, inflation, rim rigidity, and residual stress (such as within a rim that would not be true if unlaced).
It can be thought of as being analogous to placing a heavy object directly on the center of a stable arch. In fact it literally IS doing this, only 'upside-down' from 'convention', because the rim is an infinite arch, with its stability arising through circumferential compression. Instead of having its ends resting on the earth, its ends exist virtually and securely within an effectively rigid foundation (within its own frame of reference), which provides the hub as its interface point to your external frame of reference.
In this arch (your RIM) there is some inward flexure from the earth (or other source of inertia or impulse), however infinitesimal it may be, and it is this movement that tends to relax the bottom spoke(s) by pushing through them, the force submitted being equal to the total weight applied. Spoke elasticity (degree of stretching with tension) accounts for the inward rim displacement and provides a continuum (wherein a spoke's tension is reduced along with its length, to a given degree [without instantly going slack]). It is this cyclic length change that induces spoke fatigue (along with that arising from driving and hub-braking forces, which provide equal-and-opposite tensile actions in respective leading and trailing spokes).
If there is one most load-affected spoke, it would be the one closest to the point of bearing contact on the 'riding' surface. The load-carrying capacity of the wheel is strictly limited by the tension of the least-tensioned spoke in this way, because the stability of the wheel may be lost when a spoke goes fully slack, as the rim is then left on its own to bear whatever residual (a quantity of the applied force having been 'consumed' by slacking the spoke(s)) static or transient load is presented.
Bottom line, make a wheel TIGHT enough and TRUE enough, and you will not need to be concerned with relative tension. If you are, it is almost certainly the rim's fault.
Much of this has been paraphrased from my own copy of the book, which I have carefully read, and which I put to practice with 100% flawless results (notwithstanding a wrench-stripped nipple during a build here and there because of inadequate lubrication).