I have a music theory book that I study a lot and I know it well all the way from species counterpoint to secondary dominant and the Neapolitan 6th chords. I know it all except the end. It starts talking about 20th century stuff like serialism and set classes and I just don’t understand it. My question here is should I really try to learn it or am I okay with knowing 90% of the book. How vital is it to know? I don’t really even like the music examples at the end that have to do with serialism so I’m guessing it’s okay?
Let's just be clear that there is always more theory you could possibly learn, if you wanted to. (That's probably true of almost all fields of study.) An intro book is generally meant to be some sort of overview of many major ideas in music theory, but there are entire graduate courses offered regularly in music theory that it's likely your intro book doesn't even touch on.
There are also a lot of very specialized ideas in music theory which really only appear in professional academic journals. Even specialists in music theory often aren't prepared to just jump in and read an article on some wildly different kind of music analysis technique.
So, what you've likely learned so far is a smattering of the basics of tonal music theory. If you want to learn more about tonal music, for the moment I might recommend finding a more advanced book that deals with explaining and analyzing music you like.
Serialism was an interesting idea, and it's somewhat important in the history of music in the 20th century, but its relevance to new "classical" music (let alone any sort of new popular music) wanes with each passing year. Still, you could learn the basics of serialism in a few minutes if you wanted too -- and there are decent videos online if the book doesn't make sense by itself. As for set theory, it's a mathematical tool largely invented by music theorists around the 1960s and 1970s to classify groups of pitches. There's really no evidence that composers like Schoenberg et al. ever thought of composition that way, so its relevance if you actually want to understand how the music works is highly suspect. (Note: the preceding sentence is my personal view; some may disagree.) Set theory has become a default tool in music theory for classifying groups of notes in certain contexts, so you should have some familiarity with it if you ever want to dig deep into academic articles on music theory. But unless you buy into the manifold assumptions and the rather crude classification apparatus, set theory has limited practical utility beyond that, especially for tonal music.
In sum, it's always good to learn new things. But I don't see the point in trying to force yourself to learn terminology and abstract systems right now that's mostly relevant to music you don't understand and don't like. Instead, as I said, find yourself more advanced books on music you like and are interested in, and build up from there. Come back to the post-tonal analysis chapters if and when you get curious about that sort of music.
What you need to know depends on what you want to do. Much of the serial and class set stuff is not closely related to common practice or jazz or pop or Latin or country composition and performance. If you are just composing for yourself or something like that, you many not need to look at some the "newer" (if I many say newer about century-old techniques) stuff. I you want to teach music or play in a more classical-oriented orchestra, you do need to know some of the modern stuff because some students may want to learn or you may want to perform more morden works.
Of course, as you are self-studying, you can learn what you want or need now and then pick up the other stuff later if you need. I haven't found much use for lots of theory; much doesn't describe what I either play or compose or even listen to. It never hurts to learn more though. I'm not particularly sure the sounds I want to get but I haven't found set theory useful in either composition or analysis (I have found most "Western" theory from 600 AD to the present useful though.)
It sounds like you're beginning to come to terms with the language and building blocks of Common Practice music. (That's not supposed to be condescending - those of us who have been at it for 50 years are still learning!)
Now what came next? Well, Common Practice kept going, and still does, it's the basis of a lot of today's music. Some modernists who felt CP had got as far as it was going to hived off in all sorts of directions. Not just 12-tone stuff, though textbooks sometimes write as if that became the new norm. It didn't, and certainly not in any pure form. Then there's Jazz Theory, for those who like to think of Flat5 Substitutions rather than Augmented 6ths. (It's all still pretty CP though at heart though.)
Yes, you should know at least something of where music went after 1914.
It's hard to teach yourself something like this in it's entirety. These modern concepts could be based on math concepts and may or may not be mainstream ideas. I've learn a lot of theory in my life. You have to realize that humans create new ideas all the time and no matter how much you know tomorrow there will be a bunch of new stuff to learn. It is my opinion that learning more theory does not necessarily make one a better musician or composer. It can be inspirational at times but it is not a prerequisite for playing and creating. Also, some books do a better job at explaining some material and suck at other material. It could be that the author of your book was just trying to be "complete" and is not really an expert on sets etc. If you really want to learn this I'd venture out and look for other books on it that do a better job. But the answer to you question is that it is perfectly okay to not understand everything in a book. You will learn it as you go, as you integrate the idea with other things you do understand.
Why are you learning music theory? What is your motivation? What use do you intend it for?
The people who created music that was serial or set-based had studied common practice theory. However, their music operated on a different set of ideas. The theories that appear at the end of your theory text are an explanation of the building blocks, aesthetics, stylistic components, and constraints that made up this new music.
One idea that might help, is that music can either express an idea, or not express an idea, and that there are many ways to do this.