Most all of it, but you have to remember that Rimsky-Korsakov wasn't around by the time that electronic instruments were coming up. The only thing we'd sort of recognize that fits his timeline would've been the Telharmonium, and there was only one of those and it was more of an experiment than anything else. So what's necessary is to translate our sound production methods to how he would work with acoustic instruments. Fortunately, much of the timbral spectra you find in an orchestra often has parallels in electronic sound generation.
Rimsky-Korsakov was very familiar with Helmholtz's "On the Sensation of Tone", clearly...so his orchestration techniques were informed by Helmholtz's concepts regarding timbre and "clearing space" for the partials from the orchestral instruments so that everything sounds "clearer". In his time, he would work this out on paper in a score...but we would be more likely to view this process as being closer in character to mixing. So, if you pay attention to that aspect as well as his methods for grouping timbres, then yeah...this book's pretty useful. And if you side-by-side it with Helmholtz's book (https://www.amazon.com/Sensations-Tone-Dover-Books-Music/dp/0486607534), you then get a VERY clear picture of how/why Rimsky-Korsakov dealt with orchestration as he did. So, unlike prior treatises on orchestration, his actually takes cues from the emergent science of acoustics as a partial basis...which is why, once "retranslated" into our present-day electronic and electroacoustic lingo, that orchestration text "translates" very well to how we produce.