I know exactly how you feel, I tend to be good with technical stuff and lacking on the creative side . . . I need a nudge in that direction everytime, no matter the medium!! (Cooking . . . decorating our home . . . what to wear . . . and yes, photography.)
I have found a bunch of "photographer supply" sites that offer posing
cards - which would be my own preference, over a book. It's portable, you can show it to the client on location, and use it for inspiration on the go. Do a google search on Posing Cards, and I think you'll find lots of leads. Several of the top hits I got were actually forum threads discussing them, with links out to suggested products.
What I am doing personally, is sort of creating my own . . . I'm saving a private collection of great posing ideas, and just referencing them frequently. I'll build my own cards to share with clients out of my own resulting portfolio. I think a purchased set is a great solution if you have a more immediate need, though.
Also, just a tip about photographing teens . . . no matter how worried you are about your ability to pull off "natural" . . . they are feeling 100x more self conscious. It's endemic to the state of being a teenager, lol! The very best thing is to get them laughing - it creates at least a few seconds of unselfconscious body posture when that laugh erupts, and their faces look so amazing. You will both relax a LOT after that - and there is nothing more beautiful and optimistic, than someone having a laugh! Posing is not unimportant, obviously they have to hold still in some sort of fashion - but don't forget to have them cap that off with some naturalness too. Teens can be a riot, tap into that! Even snapping pics of them making funny faces (which demonstrates you're in, whole hog, too!) can lead to some giggles and that bit of bonding that needs to happen between you and the subject. I'm attaching some silly pics I shot, that lead to "that" portrait afterwards. Trust me, this is one of the most self-conscious, I-hate-my-picture-being-taken-kids you ever met.
After she was relaxed, I got this shot of her, which turned out to be The Keeper, and the main photo the family chose:
There a couple more (annotated) in
the same album, in the Get Them Laughing vein, even if those shots aren't exactly the keepers.