If you can get a copy of this book: http://www.amazon.com/Practice-Network-Security-Monitoring-Understanding/dp/1593275099/
Great info on sensor placement and NSM in general.
On network security monitoring (network ids/ips) you might want to have a look at that book http://www.amazon.co.uk/Practice-Network-Security-Monitoring-Understanding/dp/1593275099/ref=pd_bxgy_b_img_y