i have only just got a detail pen after a friend recommend using one for the model in the video and they are great! there perfect for writing on the pages of books and purity seals on space marines.
the one i got was about €3 but there are more expensive ones out there and you can get them as part of a set. there one i am using is 0.1 but there is a 0.5 that i would like to give a go. 