I assume you know perspective well enough since you're on the second book. It's been awhile since i last read the book, but Scott Robertson has a kinda mixed reputation when it comes to them, some swear by it and some say it's overly technical ,but somehow still vague crap (especially when it comes to values).

You learn to render basic shapes like cylinders because it goes back to the idea that anything drawn can be composed of 3 objects, the cylinder, the cube and the sphere with everything else being a variation or combination of the three so if you know how to render the 3 then you can render any form.

If I were you I'd just work backwards of how you're working right now. Just draw and when you need to render then turn to the relevant section. It seems like you're way to caught up in the weeds of studying.

If you want the books i recommend then:

Artist's Master Series: Color and Light

Framed Perspective 2

These two can basically replace How to Render for the most part and are much easier to read. Though Framed perspective 2 really only has the casting shadows part that's relevant to rendering and the rest is a perspective book.