For heaven’s sake, do not introduce a new, physically significant equation by saying “From Eqns 1-98, 5-24 and 11-38, we get [new equation]“. I am quite unable to follow Wangsness’ introductory E&M text because he does this so frequently. Even if one reads it sequentially, so that one has indeed encountered equations 1-98, 5-25 and 11-38 before getting to the new equation, one does not remember what they are. So one has to flip back to several previous chapters, and when one looks up equation 11-38, one finds that it in turn refers back to equations 10-56, 2-11 and whatnot. For this reason Wangsness’ book is absolutely useless if one wants a quick physical explanation of anything.
Compare this with Griffiths, who prefaces the presentation of each new equation with a lucid description of its physical meaning. I got through about half of the entire Griffiths text in the time it took me to get through one chapter of Wangsness, simply because Griffiths doesn’t require me to go sleuthing about for the justification of each equation. Also, I am much more willing to swallow the correctness of an equation if you offer me a physical justification for it there and then, instead of telling me that if you shift the quantities in some other equation about this way and take their curl, blah blah, you will get this equation.
This ties in with my general frustration about the mechanical computation that characterises most of my physics courses. I cannot watch someone calculate things for more than twenty minutes without falling asleep. More concepts, please, and less of this mechanical drilling on how we should shift variables about according to certain rules of thumb. There is no physical reasoning involved in all that. Nor any mathematical reasoning, even.
I guess this depends on taste. I try to derive each equation in the book. I am quite happy to do the mechanical computations. But I agree with you that authors should not write “equations 1-98, 5-25 and 11-38 before getting to the new equation”, they should repeat the equation. This is one of the reason I am a big fan of Srednicki’s book.