This is how I believe a Remus a Sirius got together and it does go up until book 7. Which means there is book moments too.
Year 1
James, Sirius and Peter all become friends at the start of term feast. (Well actually just Peter becomes they're friend but you get the point.) Remus doesn't befriend them until later that year but he is already friends with Lily. When they do become friends Remus is instantly charmed by Sirius.
Year 2
(Near end of year) James, Sirius and Peter find out about Remus' condition. Which of course we know that they still want to be his friend.
Year 4
Remus stars to develop...