UPDATE July 29, 2014: The ratings are back for the July 28 late night broadcasts.
NBC hasn't been releasing overnight ratings for the late night talk shows since July 15. I'm not sure why they discontinued this or if it's temporary or permanent. The ratings had gone on hiatus once before since I started this blog - May 22-25 - so hopefully this is just a temporary thing.
If anyone has an alternative source for overnight late show ratings in the top 25 markets, please let me know!