I am a Magazine Vendor in my regular job.
I love it, don't get me wrong. (Well, sometimes, I love it.)
The stores I do magazines for.

Sanford, N.C.
1. 5 x Food Lions (Including one in Buffalo Lake)
2. Walmart
3. 2 x CVS, (Magazines and Book)
4. 3 x Walgreens
5. Lowes Home Improvement (They do not recycle the mags, they force me to throw them in the trash. A very non-green store)
6. Lowes Food, (Magazines and Book)

Chapel Hill, N.C.
1. Walmart, (If I help my son, Harris Teeter and Lowes Foods)
2. Harris Teeter
3. Lowes Food, (Magazines and Book)

Holly Springs, N.C.
1. Michaels, (Magazines and Book)
2. Walmart
3. 2 x Food Lions.

Fuquay Varina
1. Walmart,
2. CVS, (Magazines and Book).
3. 2 x Food Lions.

I put out books in certain stores. CVS, Michael's, and sometimes at Harris Teeter, when I'm helping my son with his stores, which I haven't done in quite some time, and I miss working with him. I think I will go out and help him next Friday.
I brought this up because the books we put out from the authors are all the same authors, more or less.
The only thing I can think of is that they are writing the books, which possibly takes them about 2-4 weeks. They then hand it off to an editor, who edits the book, while the author is busy writing the next book.
And then once the editor is finished, the author takes the book back. Reads over it, makes changes, hands it back to the editor, who finishes it off, and submits it to the publisher.

That is the only rational explanation for the number of books these authors put out.
(or)
They have ghostwriters.

It is all speculation, and there is no way to know what these writers are actually doing.
But for me:
I handle all aspects of writing, editing, proofreading, design, Book cover creation, biography, and Photography, all of which are done solely by me.

I am not a professional editor by any means. So, I am really unsure what they do with the transcripts they are handed. Of course, they could just make corrections and nothing else. But if that is the case, how can the writer get so many books out in a short period of time?

I can type pretty fast, but I'm not really sure how many words I can type in a minute, as it's been a while since I took one of those typing tests. (About 3+ years ago), I could write with limited errors at over 125 words a minute.
So, I know with that, that I can write as much as these authors, and it takes me (with writing the initial storyline, and then the editing and adding of content, removing, and adding of pages) several months to get it just right to be published.

Oh, the wonders of authoring.
Have to love it to do what we do.
And I do love it!