Jackson's "classic" book, Classical Electrodynamics, is not among the best textbooks for this subject. In fact, there are a lot of ways in which it is a bad book because it fails repeatedly at its primary purpose, which is to teach a subject. In some ways it doesn't fail, it just doesn't do as good a job as dozens of other books. And the reason it sometimes performs so poorly at its primary purpose is because it is also designed with a secondary purpose in mind, which at times supplants its primary purpose.
John Jackson comes from the oldschool of physics, and I respect the oldschool of physics tremendously. It's how I was taught physics. His view was that physics used to be a calling, it was a subject of study for the select few, the gifted, the high-powered intellects who were born for science, who could devote themselves to it fully. He was not interested in making the world of hard science a soft place for soft intellects, for casual visitors, where the hand must be held every step of the way, and delicate people with delicate minds would be given access to labs and computers and resources they would be utterly incapable of making scientific advancements with. And his book, in his own words, is a stalwart devotee of that mindset. I think this is the right way to think about physics. I appreciate the book for that reason. John was right to act as gatekeeper. Gatekeeping was the secondary purpose of this book. But his gatekeeping sometimes got in the way of the textbook's utility. This is why its primary purpose, education, is not always so well realized. And there are many gifted and intelligent physicists out there who find the book disappointing.
For everything the book does poorly, there is something it does well, and for which it should be commended. It's a great reference book. If you've learned any part of EM properly, that is, you've learned it somewhere else first, then Jackson's book makes a really good resource for reminding you of the general mathematical formulations and low-level ideas, or expanding on certain concepts that may not have been treated elsewhere. It dives deep into corners of EM you'll probably never explore again, and it goes hard at them.
I used its section on Cerenkov radiation and a few related parts for my PhD research. I referenced it at the first job I had out of school. I even return to it occasionally now, usually for the same reason I return to most of my physics textbooks: to admire them, flip through them, read chunks of them, float down memory river, and sink into a world of beautiful science, away from the nauseating world of ideologically fueled ideas completely divorced from the kind of thinking that makes science the superior enterprise.
Another good thing about this and certain other textbooks is that if you've used them in school you will have little patience for someone who complains about being "forced" to read anything they were "forced" to read in school.