Franklin W. Dixon. Carolyn Keene. Victor Appleton. Familiar names, all three, as are their literary creations: The Hardy Boys. Nancy Drew. Tom Swift. How do these authors manage to have their names on such long-running series? (The first Hardy Boys adventure appeared in 1927; the most recent just this past February.)
The answer: these authors are all house names. That is to say, the name is a pseudonym that is owned not by the writer of a particular book, but by the publishing house, thus enabling the house to hire different writers at need for the series, or to have more than one writer at a time working on different books.
Frank and Joe Hardy (and Ms. Drew, and Tom Swifts Senior, Junior, and III) were creations of the Stratemeyer Syndicate, and their prolific adventures were made possible in part by the detailed outlines which the syndicate provided to its authors. You can read one such outline here; as someone who was one-half of Victor Appleton not once, but twice, I can vouch for the fact that the novels are still built on outlines just as detailed.
These days, the writers are likely to be given a brief plot synopsis from which they are expected to produce the outline in question, which then goes through several rounds of back-and-forth revision until it gets publisher approval. But what comes out of the process at the far end looks remarkably like that early example.
Sometimes, though, things can get weird. We – my co-author and I – once did a last-minute revision job on such a novel, for which we got the original manuscript and a copy of the cover flat (the cover of a mass-market paperback before it gets wrapped around the actual book; done as a sales tool, for handing out as publicity or showing to booksellers, it will have sales and marketing info printed on the reverse side) along with the instructions, “Fix it however you need to, just don’t contradict the back cover copy. Also, we need it in three days.”
We did it. In three days. That Victor Appleton, he’s one tough writer.
Dr. Doyle's Blog 
Reblogged this on Madhouse Manor and commented:
“My frog is dead!” Tom croaked….
Ha. That must have been quite an adventure, those three days.
I’ve read about Lester Dent, who among other things wrote at least a plurality of the Doc Savage pulps under the house name Kenneth Robeson. I think he averaged around a month or two per novel — on a manual typewriter, of course. Most were admittedly so-so, but some had some very clever and even foresighted SF concepts for the 1930s.
Dent’s outline for a 6k pulp mystery/thriller story still seems to have some juice to it, so let me link it here:
http://www.paper-dragon.com/1939/dent.html