How can you reread Mercedes Lackey every year?
I've had the chance to get some of her books cheaply a few years ago, but looking through her bibliography, she seems to write a book every other month, so I declined to get them.
Look at this
A huge number of books have a "with <writer name>" prefix. I wouldn't be surprised if these people are doing most of the writing.
Another writer I've seen who does this is James Patterson, and I won't be surprised if writers such as Daniëlle Steel do the same. Daniëlle Steel "writes" 4 books a year, and has 6 to be written and published in 2016. That's impossible. Nobody can write that much and that fast.
It could even be possible that there are a number of people writing, all under the same name, with the original author just as some sort of supervisor.