I started quilting before smart phones and when pictures required actual film that needed processing... I started keeping 3-ring albums of my completed quilts. First just pictures, then tidbits of information, then little sample blocks of the actual fabrics because they looked different than the pictures. Now I include all sorts of notes to myself -- what worked, what would have worked better next time, and any templates I may have made. I also include the pattern whether it was one I drew up myself or bought -- I found it much easier to find what I was looking for if it was all in the same place. For books, a quick photocopy of the project with the name of the book. The "loose" stuff goes in a plastic baggie and can just be punched through the rings of the notebook.
I also take various pictures (which I periodically store/backup) that I call "process" pictures. I don't always print these out but in addition to the notebooks, I have computer folders for each project.
I find if I have to put away a project before finishing that a note to myself telling why I stopped and where I am at -- like "need 23 more 9-patches" or whatever helps quite a lot!
Some of my projects require collecting fabrics for long times, often years. I make "project boxes" to keep everything together. I include the pattern, notes to myself, and at least a sketch of what I am trying to achieve.