I make extensive notes on the material I encounter all over the Internet (video courses, textbooks, blogs etc). I want to make my hard earned notes available free over the Internet. But if I do that, then it won't be fair as everybody will want to read the notes and won't buy textbooks.
Is it copyright violation if I share my notes over my blog?
Needless to say, this is not legal advice about your specific situation, and if you're really concerned about this, you should consult a lawyer.
However, here are the general rules. (Law professor here.)
Copyright protects expression, not content. So if I read a textbook in order to learn something, and then summarize it in my own words to share that knowledge with the world, I own the copyright to the summary, the textbook author does not.
Not all text is copyrightable. In particular, text that just represents information that can only be expressed one way isn't copyrightable. The classic example of this is a telephone book (see Feist v. Rural Telephone Services). But there are academic examples that would fall in the same category. Off the top of my head, an obvious example would something like the information in a table of mathematical results (although if the table is in a novel structure, the information as laid out in that structure might be copyrightable).
Not all information that is copyrightable is in copyright. For example, the formulas in a math textbook for a subject that's been around for a while may not be in copyright because whoever first wrote them in that form did so a very long time ago, and copyright is time-limited.
Finally, there's fair use. That's a super-complicated subject, but broadly speaking, noncommercial uses of small amounts of copyrighted work tend to be ok for purposes like teaching and critique. But for that, it's best to do lots more reading and tread carefully. More information.
If your notes are entirely your own writing, you're probably fine. I.e. if you summarize, paraphrase, and the like, then you shouldn't have much to worry about. The fact that your notes might substitute for book itself is just the facts of life in the marketplace of intellectual ideas.
All that being said, if you copy literally from the book, you might find yourself having to deal with legal threats, DMCA takedown requests, or an actual lawsuit. None of us can (or should) tell you how that might go. You should consult an attorney before you post anything if you intend to copy liberally from the original sources. You may have a number of defenses if legal process should commence, but that doesn't help you save on attorney's fees and other costs. So be careful that all your words are your own.
