Resist is a great re-introduction into the world of Possession, as it has been some time since I've read it (over a year).
The story in Resist takes place before Possession. Even though it is a prequel, I'd highly recommend reading Possession before
diving into this, or else you might find yourself a bit confused.
Resist is only around sixteen pages (the first chapter of Possession is included at the end), but even though it is short, it's nice to learn more about Zenn. I wasn't the biggest fan of Zenn in Possession, so it was nice to learn more about who he was before.
My review for Possession can be found here: http://yabookhaven.wordpress.com/2011/06/16/possession-possession-1-by-elana-johnson/