If you've some time to burn, write the author and/or his publisher and let them know that the guy's new ghostwriter sucks shit. If this is very seriously making your consider not picking up the next book in the series, be sure to mention that.

If folks just stop purchasing the new books, they can imagine a reason for the lost sales that's convenient for them, but if folks tell them why they stopped purchasing, there's a lot less room for that kind of nonsense.