I use a blank paper sheet as bookmark and carry a pen, so that I write down important notes from the book content on the bookmark. Afterwards the book is - ideally and dependent on the content - condensed to one sheet, that I can refer to later. It also helps with retaining the content.

I’ve tried this with an index card once and quickly filled it up. As I ran out of space I found myself question if someone was card worthy, which was a distraction.

I also tried writing in the margins on a book. I don’t like writing in the book due to it losing a lot of ability to shared after the fact, or if the book isn’t mine it’s a non-starter. That said, it was a very different experience, it was more like having a conversation with the author.

I may need to experiment more to try and find something that works.

I have also not found a great way to take notes while reading books, but most of my reading is on my computer right now. That makes digital note taking the obvious choice, and space isn't really an issue in that case. My way of deciding what to take notes on is by simply asking myself: could I want to remember this in the future? If the answer is yes (it usually is if I even considered asking it) I make a note of it. The note minimally contains a reference to the source, and some links to related topics. The links to related topics allows me to discover the note in the future by referencing the backlinks on those topics.

I recommend looking into the Zettelkasten method for note taking, if you are not already familiar. It is originally used with index cards, so you might find it to be a useful way to handle taking notes on index cards. I use a Zettelkasten-inspired digital note taking system called Org Roam. The thing I like the most about Zettelkasten is that it minimizes the effort for getting your ideas out of your head. You can worry about organizing the ideas after you have them laid out in front of you.

I use Obsidian at work to organize and plan. When looking up ways to effectively use that I stumbled down the Zettelkasten rabbit hole. It’s a little too involved for my needs. It seems like it might be good if I was doing research to write a book, but is too involved for my daily life. I can see where taking and organizing the notes would turn into a hobby, which is enough of a problem for me already.

I’ve considered reading on the computer, maybe I should actually give it a try.

> I’ve considered reading on the computer, maybe I should actually give it a try.

I should note that I haven't actually gotten into a workflow where I read ebooks and take notes. Most of the reading I do is documentation for systems I program with. I take tons of notes on documentation so I can easily find what I need again in the future.

> I found myself question if someone was card worthy, which was a distraction.

I think having a whole sheet might help. It's not enough to copy everything, but enough not to get sidetracked. Also, you might write down a reference to a page and 1-2 keywords only, or a couple of keywords describing one's own thought regarding a piece of content + a page reference.

You could try searching for "Post-it Flags, 200/High Volume Desk Grip Dispenser"

* They remove cleanly, if you're worried about permanently marking up or defacing a book.

* They are easy to write on (I've mostly used EF nib fountain pens).

* Best of all, since they are color-coded, you can invent your own scheme (e.g. blue == "I should revisit this", yellow == "my top N favorite quotes from this book", etc.)

Edit: to add another point

* They are large enough that you can write a decent sentence in there -- the "page flag" size might be a tad too small for that (but thanks to the EF nib, I often find myself using different-colored flags).