kengo has a pretty good idea in his Flickr. While reading Getting things done, he keeps a piece of paper which he can write notes and also use that as a bookmark. You will need to have a smaller size of paper to able to do that.
Bookmark in a book – [kengo's Flickr]
lifehack: Notepaper as a Bookmark
lifehack brings us yet another great idea: notepaper as a bookmark. [here]
Before I started using 3X5 index cards, I bought a ahck of 4X6 cards. Turns out that the 4X6 cards don’t fit in the Mead Hipster PDA case (here and here). But now, I think I…
I prefer using Post-it notes. I keep 2 stacks of 10 Post-it notes in my Hipster PDA. Nice, bright yellow–great for writing down quick notes or using as bookmarks, and they won’t fall out just in case you open the book the wrong way.