One on one training has always been useful in any discipline. Just getting feedback and correction is enough reason to justify a private lesson or two. The lessons are costly, but you will improve much faster. The cost is at least partly offset by the fact you will benefit more from your regular lessons.
Seminars and Camps
Perhaps the opposite approach to private lessons, but this is very different to regular classes. You learn from a different teacher, you interact with different people; you won’t get stuck in a routine. And you tend to get quite a lot from one weekend away.
Ok, this presumes you have some level of competence. It is terrible when the beginners learn mistakes from the instructor. But teaching puts things in perspective. There is a tradition of professors often teaching the introductory class. This is partly to prevent the beginners acquiring misconceptions, but it also lets the teacher keep some perspective; they remember what it was like to acquire these skill the first time; the remember that the subject does not exist in a vacuum. Teaching reinforces the foundations of knowledge.
The only issue with books is finding the good ones. Recommendations help; read Amazon reviews. The cost of a book is often less than a lesson or two. And even if you only get a few solid ideas out of a book it was worth the price.
On line Training.
If you already have the internet, and most of us do, it doesn’t cost anything to look up some instructional advice. Again, there is good advice and mediocre, so finding the worthwhile karate videos is a hassle. But this is far better than 99% of the time we spend surfing the net. If you find a good site, tell us about it.