My feedburner email subscription stopped updating one day - i.e., the daily email stopped going out. If you have this happen to you as well, one thing you should check is whether your feed size is over 512K.
Note that Feedburner would not let you setup a feed with a size greater than 512K, but if your feed fluctuates above and below that cutoff - the subscription will fail without warning when your feed is above that size.
To reduce the size of the feed, I have to use the “max-results” option to reduce the amount of posts. I.e., http://kindlebooklist.blogspot.com/feeds/posts/default?max-results=2.