The paper is good quality- I tested a black Triplus on them and it didn't go through to the other side, but you could use Sharpies or anything really as each page is one-sided.
Here are my finished pictures, I hope you like them! I really want to frame these as it's exactly my type of thing- slightly strange and gothic. I used my Faber-Castell art pencils with a bit of Triplus on the details. Watch this space for a review of the sequel to this book: Monsters!