Hmmm, as far as I know, Amazon still supports Whispernet and delivers books over 3G if you have a model that supports it. Kindles can also use Wi-Fi. I've given Kindles as gifts, but as a neo-luddite that prefers to turn paper pages, I don't have one myself at the moment, so I can't check this right now, but I wouldn't be surprised if a 3G/WiFi Kindle automatically uses Wi-Fi if available instead of 3G. Could that be what you are seeing?
If your Kindle was receiving books over 3G and that has stopped, I'd contact Amazon's Kindle customer service with by email at email@example.com or by phone at 866-321-8851. Amazon is one of those increasingly rare companies that still has good customer support, so they should be able to help you out.