I suggest you try looking at some (ok, better a lot) of nudes to see what kind of poses and angles you and your wife prefer. I found that when taking nudes there are fewer universal rules as it is a somewhat taboo theme, so just feel free to play with your imagination. Many people dislike wide angles as they tend to distort the proportions too much, but on the other hand some people like them for exactly the same reason (can be useful to make the legs look longer i.E.).
Best is to think about the shots before the actual session, draw or write down your thoughs and make a plan of exactly what and how you want to take it. This will of course be altered here and there as you go, but I suggest you have some general guidelines and directions before you start.
You wife needs to get undressed and get in a bathrobe ca. two hours upfront for the underwear traces on the skin to disappear. Therefore if you are taking both nudes and semi-nudes in one sessions, start with the full nudes. The most important thing during the session is the comfort of your wife. The more comfortable she is, the more relaxed she can feel, the better and more natural photos you can take.
Some books I found helpful for the nude photography beginner:
[ame="http://www.amazon.com/Nude-Photography-Craft-Pascal-Baetens/dp/0756631769/ref=sr_1_1?ie=UTF8&s=books&qid=1232581868&sr=8-1"]Nude Photography: The Art and the Craft, Pascal Baetens[/ame]
Has a bit of all: history, lightning, posing, mini-portfolios. A nice overview of some nude photography basics. Nothing spectacular, but quite useful.
[ame="http://www.amazon.com/Lighting-Nude-Photography-Professionals-Secrets/dp/294037810X/ref=sr_1_2?ie=UTF8&s=books&qid=1232581992&sr=1-2"]Lighting the Nude: Top Photographers Professionals Share Their Secrets, Roger Hicks and Francez Schultz[/ame]
This book is fool with photographs and small lightning graphs. Good for inspiration as well as a guide to learn about other photographer's lighting.
The best thing however is to look at many many nude and semi-nude photos, alone or with your wife, and see what you like and would like to try. For a beginner there is no shame in drawing inspiration from other's works, or even trying to mimic them.