There is a list of recommended training and study resources on the Study/Learn page for the IINS (CCNA Security) exam. Please click here.
Cisco Learning Network Moderator
I have not taken this exam, but I have looked at the materials quite a bit. The thing that I found interesting is that it doesn't really seem to touch on the ASA at all. I would have expected it to since that is Cisco's flagship security product. I guess that would be a lot to introduce at that level and in an enterprise might not have made sense anyway. However in the smb space, people wear a lot of hats and that often includes the Firewall. As for materials, I think the following Cisco Press book by Catherine Paquet looks pretty good.
I've not read it cover to cover, but did look through a lot of it.