If you like physical books, you could use sticky notes to annotate books without marking the books. I've used this Traveler's notebook insert for several years to annotate library books:

https://www.amazon.com/Midori-Travelers-Notebook-Refill-Sticky/dp/B00K71AA52

... with that being said, I've started to perceive books as a fairly poor medium of knowledge exchange. Even my best read friends tend to have difficulty internalizing or otherwise remembering the contents of the books they read. The issue is that the metacognitive skills required to reflect and ask yourself the right questions in order to solidify knowledge are fairly absent in most people, and even in the people who do have those skills they're very taxing--it's very difficult to determine which questions to ask in a field you're an amateur in. I tend to remember books of the best of my peers, but it's because I talk incessantly about whatever I'm reading to my friends until they're sick of me.

I also have tried to maintain corpuses of notes or wikis for knowledge generation and I always end up abandoning them. It takes a lot of effort to maintain those kinds of collections. I'd love to hear from those who have actually been able to keep these personal knowledge collections up-to-date and personally useful.

What I'd be interested in is which other models of knowledge accumulation people here have found effective outside of traditional models of learning through either traditional education or books. Specialized wikis are my favorite ways to accumulate knowledge so far (e.g. https://supermemo.guru/wiki/SuperMemo_Guru ), but they're few and far between.